Comparison HP 9845 vs. Apple II, Commodore PET 2001 and Tandy TRS-80

it was the year Star Wars and Saturday Night Fever hit the cinemas. And Fleetwood Mac released its Grammy-winning Rumours album, the Clash released their first album, and it is the year when Elvis Presley, the King of Rock 'n Roll died.

And several outstanding computers were released, including the Apple II, the Commodore PET, the Tandy Radio Shack TRS-80, and, of course, the HP 9845.


So what's more obvious but to start a comparison of those systems, and see what made them successful. Each with its own features.

Comparison of Technical Data

  Apple II TRS-80 PET 2001 HP 9845
Name Apple II TRS-80 Model I PET 2001 HP 9845A
Manufacturer Apple Inc. Tandy Corporation Commodore Business Machines Inc. Hewlett-Packard Co.
Type Home computer Home Computer Home computer Workstation
Origin U.S.A. U.S.A. U.S.A. U.S.A.
Release Date April 1977 August 1977 September 1977 September 1977
End of Production 1980 1981 1982 1980
Number of units produced 40,000 More than 250,000 unknown unknown
Successor Apple II+/europlus (1979) TRS-80 Model III (1980) CBM 3032 (1979) HP 9845B (1979)
Built-in Language Integer BASIC (Woz' BASIC) Level I BASIC (based on Palo Alto Tiny BASIC), later Level II BASIC (based on Microsoft BASIC) Commodore BASIC 1.0 (based on Microsoft BASIC) HP BASIC
Keyboard Full stroke, 52 keys, caps only Fully stroke, 53 keys, caps only, later with 12 key numeric keypad 73 key 'chicklet' keyboard with numeric keypad Full stroke, 120 keys, (84 standard keys with numeric keypad, 36 special keys)
CPU MOS 6502 (8-bit) Zilog Z80 (8-bit) MOS 6502 (8-bit) 2 x HP Hybrid CPU (16-bit), nanoprocessor (8-bit) for internal printer
Speed 1 MHz 1.77 MHz 1 MHz 5.7 MHz (hybrid), 8 MHz (nanoprocessor)
Maths Software (integer only) Software (6 digits Level I BASIC, 16 digits Level II BASIC) Software (10 digits) Hardware (12 digits)
Complexity Single 2-layer PCB design (mainboard), 85 ICs (48 kByte RAM), all ICs socketed, separate switching PSU Dual 2-layer PCB design (keyboard and logic PCB), 39 ICs, all TTL chips and static RAMs soldered, sockets for DRAMs, ROM and processor), PSU on mainboard with separate transformer Dual 2-layer PCB design (mainboard + CRT) 70 ICs (8 kByte RAM), all TTL chips directly soldered (sockets for RAM, ROM and processors only), PSU on mainboard with separate transformer Design with 33 4-layer PBCs (20 mainframe, 5 PSU, 6 CRT, 2 tape), 500 ICs (162 LSI, 338 TTL) plus 10 ROM modules, only ROMs and nanoprocessor with sockets
RAM 4 kBytes (48 kBytes max.) 4 kBytes, 16 kBytes 4 kBytes (early version) then 8 kBytes, plus 1 kByte video ram 24 to 72 kBytes
RAM Types 2104, 4116, 2116 (32 sockets), all DRAMs 2104 or 4116 DRAMs (main memory), 2102 SRAMs (video memory) 6550 SRAMs, 2114 DRAMs (8 sockets) 2114 DRAMs
ROM 12 kBytes 4 kBytes (Level I BASIC), later 12 kBytes (Level II BASIC) 14 kBytes 128 kBytes
Programming Languages Integer BASIC in ROM (30 statements and 8 functions), Mini-Assembler in ROM, later Applesoft BASIC in ROM, also PASCAL, PILOT and FORTRAN on floppy disk BASIC in ROM (23 statements and 5 functions for Level I BASIC, 27 statements and 34 functions for Level II BASIC), also some PASCAL and FORTRAN on floppy disk Adapted Microsoft BASIC in ROM (35 statements and 26 functions) HP BASIC in ROM (130 statements and 45 functions, 19 operators), entensible with plug-in ROM modules, also PASCAL, FORTRAN, FORTH by third party on floppy disk
Text Modes 40 x 24 / 80 x 24 (with 80 columns card) 64 x 16 40 x 25 80 x 25
Graphics Modes 40 x 40-48 (16 colors), 280 x 192 (4 and later 6 colors) 128 x 48 None 560 x 455
Text Colors 16 Monochrome Monochrome Monochrome
Character sets US ASCII (upper case only) US ASCII (uppercase only), 64 block graphic characters Full US ASCII, 64 graphics characters Full US ASCII, international, 32 graphics characters
Character matrix 5 x 7 (within 7 x 8 matrix, upper case only) 5 x 7 (within a 6 x 13 matrix) 5 x 7 (within 8 x 8 matrix) 7 x 9 (within 9 x 15 matrix) with true descenders
Text attributes Normal, inverse, flashing Normal (no attributes) Normal, inverse Normal, underline, flashing, inverse, or any combination
Sound Single channel (by software) None None Beeper
Size / Weight 18" x 15.25" x 4.5" (45.7 x 38.7 x 11.3 cm), 11.5 lbs / 5.21 kg CPU/keyboard 16.5" x 8" x 3.5" (41.9 x 20.3 x 8.9 cm) 4.6 lbs / 2.1 kg, video display 16.5" x 13.5" x 12" (41.0 x 34.3 x 30.5 cm) 16.5” x 14" x 18.5” (42 x 36 x 47 cm) 50 lbs / 22.7 kg 19" x 18" x 26.3" (48.3 x 45.7 x 66.7 cm), 64 lbs. / 29 kg (without internal printer)
Monitor Composite monitor (NTSC) ot TV set (with optional RF modulator) Video monitor 12" black and white (modified RCA TV) or green (Sanyo) Internal 9" TTL monitor (black & white) Attached 12" monitor (black & green)
I/O-Ports Video out (composite), 8 expansion slots, tape recorder (1,500 bps), game controller/paddles with 3 TTL and 3 analog inputs, plus 4 TTL outputs

Video out, cassette recorder (250 bps/500 bps), Expansion Interface edge connector

Video out (TTL), IEEE 488 (edge connector only, no GPIB standard jack), parallel port, second cassette port for external cassette recorder, ''user port'' for 8-bit I/O, system bus 4 I/O expansion slots
Integrated Peripherals None None Tape recorder (datasette) High speed cartridge tape drive, thermal printer
Standard Expansions Disk drive controller, super serial card, 80-character-card, Z80-card Expansion Interface: 32 kBytes memory, parallel printer, two cassette recorders, floppy (Shugart) interface Memory expansion Graphics option, second internal cartridge tape drive, internal thermal printer, memory expansion, option ROMs, several interfaces
Power Supply Switching PSU Linear PSU (transformer) Linear PSU (transformer) Switching PSU
Power consumption   ca. 50 Watts (computer)
ca. 50 Watts (video monitor)
  Max. 275 Watts
Price $1,298 (4 kByte version)
$2,638 (64 kByte version)

$399 (4 kByte Level I version)
$599 (4 kByte Level I version, including 12" monitor and cassette recorder)
$849 (16 kByte Level II version, including 12" monitor and cassette recorder)
$299 (expansion unit)
$597 (expansion unit with 32 kByte RAM)

$495/$595 (4 kByte version)
$795 (8 kByte version)
$11.500 (9845A)

Sales figures showed, that during the early years from 1975 until the appearance of the IBM PC, the market was dominated from 1977 by the "1977 Trinity" of Apple II, PET 2001 and TRS-80), until Atari joined in 1979:

Sales figures PC market

Sales Figures PC Market 1975 to 1981 (In Thousands of Units)

The apparently low relevance shown in the diagram for the Altair does not reflect the already existing, vivid ecosystem with hundreds of companies and products around the S-100 bus and 8-bit processors (mainly the Intel 8080 and the Zilog Z80). That ecosystem was the nucleus for many of the innovations which build the fundament for the computer landscape as we know it today, including mobile devices. Although utilizing another processor, the MOS 6502, none of the Apple, the TRS-80 or the PET would have been possible without those innovations, all three can be considered as descendents of the developments around the S-100 bus.

The year 1975 in fact was totally dominated by the Altair and its clones (hard to identify in the above chart because of scale). But everything changed in the year 1977, where the PC market exploded - simply because ready-to-use systems had been available, for an affordable price tag. It also shows that it is a myth that Apple dominated the PC market from the very beginning. Actually, the (by far) most successful PC during 1977 to 1980 had been the TRS-80 line of computers.

Apple II

Apple II

Apple II

After starting with the first Apple product, the Apple-1, as a bare board, the Apple Company quickly released their first fully functional personal computer, the Apple II. In contrast to later models like the Apple II+/europlus, the Apple II did not yet use AppleSoft BASIC, but rather Steve Wozniak's original Integer BASIC, and was available in configurations with less than 48 kBytes RAM.

Woz and Jobs with Apple-1

The Apple-1 actually was some kind of proof-of-concept - combining the 6502 reference design with an extremely limited, terminal-style video implementation (similar to Don Lancester's TV Typewriter of 1973), a setup, which however had been revolutionary and innovative at the time. It was built in small numbers (up to 200 were produced) and quickly replaced by the much more capable Apple II. The Apple II was really everything Steven Wozniak was able to efficiently design with the technology of the time, for a cost which was affordable for consumers, and became a tremendous success.

Today's nimbus of apple products already has its roots in the Apple II design. It definitely had been one of the best industrial designs for home computers at the time, and - although as many features as possible were implemented in software and not in hardware - it was not a cheap design. Examples were the modern switching power supply (when most other home computers - if a PSU was included - used a linear power supply with heavy transformers), sockets for all ICs (even the standard TTL chips), 2-layer mainboard PCB design, eight expansion slots with edge connectors, and a full-stroke keyboard.

And it had been designed as an open system. Due to the eight expansion slots and available specs (including schematics), third party hardware was developed quickly. However, it also did not last long until the first clones appeared (whether licensed by Apple Inc. or not).

On the other side, the Apple II was ready to use after a purchase only if you owned an (at the time quite expensive) monitor with composite video input or ordered an RF modulator for connection to a TV set at extra cost. Also, there were no real interfaces for peripherals included in the standard configuration, except for game controllers, video interface and external tape cassette recorder.

Typical for the Apple II was the design of the floppy drives. For the Apple Disk II, Apple simply packed the plain Shugart SA 390 floppy disk drive hardware with any logic stripped off and without PSU into a small box, designed a minimalistic controller (more or less for electrically bridging to the system bus), and did the rest in software. As a result, the floppy drive provided 113 kBytes storage for $595, controller included. Because of the software control of the disk drive, software vendors could implement sophisticated copy protection schemes, which were almost impossible to hack.

Apple II Ad 1977
First Apple II Ad

And the Apple II already had color and sound. It seems that the Apple II had been the first integrated commercial system offering bitmapped color graphics for consumers. I guess the Compucolor 8001 had been the first commercial desktop color computer around December 1976, which however provided 160 by 192 pseudo-pixels with graphics characters, similar to the later trs-80, but with 8 colors and including 19" color display for $2,690. Also, the Cromemco Dazzler board which was released in 1976 for the MITS Altair already implemented frame buffer based color graphics with up to 16 colors with 64 x 64 pixels or two colors with 128 x 128 pixels. But as an add-on, not as an integrated system.

In fact the color implementation was some kind of hack, working with the NTSC system only. The Apple sends a short color burst on each scanline, which mimicries color mode to any NTSC monitor or TV, and then simply uses the pixel information as phase coded color signal. As a consequence, even the high resolution graphics mode could use color, without requiring extra memory for the color information. The sound circuit simply triggered a click on a speaker by writing to a defined memory location, which then could create tones and noise with an appropriate machine language routine. Analog input devices such as paddles and joysticks where sampled with another simple circuit, which just measured the time needed to charge a small capacitor.

Finally, the Apple II wedge-style appearance (designed by Jerry Manock) with its clear lines (including the vents) and proportions and the colorful logo simply was appealing. The first Apple II advertisement shows the Apple II in a home kitchen, not really conceivable for an HP 9845A or PET. And according to Michael Moritz in "The Little Kingdom", Jobs wanted to model the case for the Apple after guess what? After those Hewlett-Packard used for its calculators. He admired their "sleek, fresh lines, their hardy finish, and the way they looked at home on a table or desk..".

Apple Inc. did have a perfect promotion strategy: first the technical aspects were published as an article written by Steven Wozniak in the May 1977 issue of the BYTE magazine, and from that on every BYTE issue included a two or three full page color advertisement showing smiling people (like the aforementioned one with the kitchen) in it. Of course the guy on the ad obviously uses the Apple II for something serious (indicated by the uncontroversial line chart diagram...).

And color ads were quite uncommon for computers at the time. Just one year later, Apple already titled the advertisement with "Why Apple II is the world's best selling personal computer".

The Apple II shows to be surprisingly stable, even after more than 30 years, most systems still work.



Click to enlarge The Apple II
Click to enlarge Front view of the Apple II.
Click to enlarge Rear view of the Apple II.
click to enlarge The back of the Apple II featured the power jack (switchable between 110V and 220V AC), the power switch, three large and one smaller cut-outs for guiding cables out of the case (the latter for the floppy cable or paddles), the audio recorder input and output, and the composite video output connector.
Click to enlarge On the left of the backplane the Apple II provides a cinch connector for composite video and two jack plugs for the data cassette recorder.
Click to enlarge See the AC power jack with power switch and 110/220V voltage selector.
Click to enlarge

Apple II with original green monitor. The color of the plastic once was the same, however not the plastic itself. Obviously the monitor case suffers much more from ageing than the Apple II case (the early Apple II was painted).

At the time the Apple II was introduced, Apple did not offer an own video monitor solution. The one showed here actually was designed for use with the Apple IIe.

Click to enlarge Apple II with original green monitor and Apple Disk II floppy drives.
Click to enlarge

Apple II switched on. The lower left light is just a power-on indicator (no key).

After powered up, the original Apple II started in machine language monitor, not in BASIC (probably this was more convenient for Steve Wozniak who presumably never used BASIC on his own). You had to jump to BASIC manually with CTRL-B or CTRL-C.

The random looking characters initially showed appear a bit weird, but it is not a defect, the video RAM simply isn't cleared during startup.

Click to enlarge

The Apple II keyboard is a full-size full-stroke typewriter keyboard, however quite minimalistic (only alphanumeric keys and the most important control keys, not even half the number of keys of a HP 9845A).

The SHIFT key had limited functionality (lower case characters were not yet supported on the Apple II).

Click to enlarge Side view of the Apple II keyboard.
Click to enlarge Apple II monitor power switch.
Click to enlarge Brightness knob of the Apple II monochrome monitor.
Click to enlarge Another detail with monitor logo.
Click to enlarge Nice close-up of the original Apple logo on the Apple II monitor.
Click to enlarge Apple II monochrome monitor. Note the pivot adjustment of the screen for optimum view angle.
Click to enlarge Note the original Apple II logo differs from the successor Apple II Plus (the small green "plus" is missing).
Click to enlarge The Apple Disk II floppy drives were stripped down Shugart SA 400 with most logic removed and done in software. So the drives are connected with the Shugart bus ribbon cable to the floppy controller in the system.
Click to enlarge Detailed view of the original Apple Disk II.
Click to enlarge

Inside view with cover removed. The mainboardis populated with some common expansion boards. From left to right: 16 kByte RAM "language card" expansion, 80 column board, Z80 board and dual floppy controller board.

The top cover is only fixed by a Velcro strip, so the Apple II internals can be accessed without tools.

The system is cooled by convection and works without fans.

Click to enlarge

View with all expansion boards removed. To the left is the switching PSU, which also supplies the eight expansion slots in the rear. All components are located on a single mainboard PCB.

The upper row of ICs (with two sockets free) is the firmware, the three rows in the marked rectangle are the RAM chips (here type 2116 for 48 kByte). Note the upper left RAM chip is left out for connecting a 16 kByte RAM expansion on this socket.

Click to enlarge View inside the Apple II from the back of the computer. See the PSU on the right, and the main LSI chips in the center (with the 6502 processor across. DRAM chips again inside the marked rectangle. Also see the colored keyboard ribbon cable and connector, and the system clock 14.318 MHz quartz in the upper right corner.
Click to enlarge

Comparison of the Apple II and the PET. Note the totally different design. The PET comes with integrated monitor capable of 40 x 25 characters display, whereas the larger Apple monitor is an extra item capable of 80 x 24 character display.

Also the Apple II has no integrated mass storage storage, however it had been quite common to add Apple Disk II drives to the standard Apple, which are shown here.

The PET has a calculator keyboard with separated numeric block, the Apple II already has a (minimalistic) typewriter style keyboard.

Click to enlarge Comparison of the Apple II and HP 9845A base systems (called 'mainframe' for the HP 9845A). The Apple II is only 70% of the size of the HP 9845A.
Click to enlarge Same comparison with monitors attached.




TRS-80 Model I Level II With Green Video Monitor
Source: Wikipedia

Originally introduced as the Radio Shack Micro Computer System, the TRS-80 Model I was the most successful personal or home computer before the Commodore C64. Somewhat strange that those breadbox designs had been that successful, but, as a matter of fact, people seem to like that kind of design. Actually, the TRS-80 with its "Mercedes Silver" or "Battleship Grey" finish was a real child of the seventies. Today we probably would call that design kitchy, but in the 70s, when metallic varnish was freshing up the car industry, this simply was freaky and cool. And, in contrast to other plastics of the time, the TRS-80 today still looks like the first day, without yellowing. The only tribute to time is that the silver paint gets worn off at the parts of the keyboard casing where the heels of hands reside.

In fact the TRS-80 was a masterpiece of economic design. It featured a full stroke professional keyboard plus monitor for just $599, which was significantly below the PET (which offered a monitor but with a chicklet keyboard) and the Apple II (which had a full-stroke keyboard, but no monitor). Probably the dominant reason for the high sales numbers. But also Tandy Corporation with its thousands of Radio Shack stores had been present everywhere, so it was easy to try and acquire a TRS-80. Last but not least, there was a 10 million existing customer base.

The first TRS-80 Model I computers were shipped with a 53 key keyboard, 4 kByte ROM with Level I BASIC (derived from public domain Tiny BASIC) and 4 kByte RAM. An enhanced Model I later was available with the addition of a numeric keypad, 12 kByte ROM with Level II BASIC (licensed from Microsoft), and 16 kByte RAM.

Its technical design went back on the work of Don French and Steve Leininger, who considered a preassembled unit more desirable than a kit. And it did some things different than Apple or Commodore. Unusual was the display format, featuring 16 lines of 64 characters (same video setup as the semi standard VDM-1 S100 video solution for Altair and IMSAI). So there were less lines but with more characters, which was advantageous for text processing and programming, while with the resulting 6 MHz bandwith requirement still feasible for using a standard, cheap black & white RCA XL-100 TV assembly with just minor modifications. It didn't provide character attributes or grafic characters, but it used half of the character set for block graphics. It used a Z80 processor rather than the wide spread and cheap 6502, or the also common, but more expensive 8080 or 6800.

For achieving a balance of features and costs, the TRS-80 incorporated some compromises. So the standard video monitor had been a stripped down, cheap RCA black and white monitor, an eighths SRAM chip for the video memory was saved for cost reasons (preventing using the patterns of lower case letters already built into the character generator ROM), and the base system did not provide any I/O port except the cassette recorder interface. The first Level I BASIC was very limited, however already incorporated support for single precision floating point arithmetic. But overall, the TRS-80 featured a convenient, compact design.

The computer itself fitted completely into the keyboard housing, but there was no space to add any extensions internally. Rather, an Expansion Interface unit was offered, which provided options for more RAM (up to 48 kBytes in total), a clock circuit, a parallel printer interface, a single general purpose expansion slot (normally occupied by the optional RS232 interface) and support for up to four floppy disk drives. The Expansion Interface was used as a base for the video monitor and connected via a ribbon cable to the main CPU/keyboard unit. The Expansion Interface and the CPU/keyboard unit both used their own power transformer, and the expansion interface had a compartment for stowing both transformers.

Some flaws in the very early circuit design could be annoying in the early times, but got fixed during the further development. Among those weaknesses were bad contacts (the TRS-80 used tinned contacts at its edge connectors), bouncing keys, and weak TTL drivers, causing some instabilities when accessing the extension RAM in the Expansion Interface. The total lack of RF shielding and several (also unshielded) cable connections caused interference with radios and TVs, and finally lead to discontinue production in January 1981 with the 1980 revision of the FCC regulations coming into effect. The Model 1 Level II in combination with a redesigned Expansion Interface was a quite complete and stable system, with lots of features for a still competitive price.

It is not widely known today that the TRS-80 also played an important role for the development of digital music. With a third-party hardware extension, the Orchestra-80/85 by a company called Software Affair, the TRS-80 was able to be used as a 5 voice 8-bit mono (Orchestra-80) or stereo (Orchestra-85) synthesizer. Although the hardware was designed extremely simple with a bus buffer, a resistor cascade DAC and a small analog amplifier, the software was extremely capable. Built-in support for modem connections enabled the users to exchange their creations via Compuserve, so there were thousands of music works available.

TRS-80 The TRS-80 Model I Level II
More pics to come ...  


Commodore PET

PET 2001

PET 2001

Together with the TRS-80 and the Apple II, the Commodore PET had been the first personal computer manufactured in series. It featured 4 or 8 kBytes RAM (upgradeable to 32 kBytes with extension board), Microsoft BASIC in ROM and an IEEE488 interface for connecting peripherals. In contrast to the Apple II, it already included a CRT monitor and a tape cassette drive, and therefore was immediately ready-to-use after purchase.

And it was half the price of an Apple II. With some compromise, like the calculator-type keyboard, and the lack of sockets for all TTL ICs and expansion slots. Also, the PSU seemed to be a bit low dimensioned, so many PET users had problems with it.

The industrial design of the PET differs highly from that of the Apple II. Whereas Apple used a fancy plastic housing, Commodore designed its PET with sheet metal. Curious for the PET design is the way how the system can be opened just like an engine hood, and as such can be hold open by the use of a rod. All together, the PET was a nice piece of hardware for an almost unbeatable price. For me personally, the PET looked like borrowed from the 2001 Space Odyssey movie. In fact another HP 9845 model, the HP 9845C had its code name 'Odyssey' from the movie, but the PET design is much closer to it. Another funny thing is that HP called its workstations 'calculators', whereas Commodore named them 'computers', but supplied them with a pocket calculator keyboard.

The appearance of the PET with its sheet metal design was not as elegant and consumer-friendly as that of the Apple II with its molded plastics, although with concerns to EMI protection much better shielded. However, with its tetrazoidal monitor on a neck, which again based shoulder-like on the main casing, the PET looked somewhat anthropomorphic.

Commodore did not offer a comparable cheap solution for floppy disks like the Apple Disk II. The CBM 2040 dual disk drive was - although using the same Shugart SA 390 floppy drive hardware as the Apple Disk II - in fact a complete dual-processor computer on its own, featuring a complex IEEE488 interface and DOS in ROM, and sold with $1,295 for twice the price of the PET. Floppy disk storage with 170 kBytes per disk was somewhat more than the Apple Disk II, since Commodore used an adapted but unique (and therefore incompatible to anything else) sector partitioning for the CBM 2040.

Today, many PET systems suffer from defective ROM chips (similar to the HP 9845B series). Since the PET case is painted sheet metal instead of molded plastics, it keeps its original white color even after more than 30 years.

Click to enlarge Front view of the PET 2001.
Click to enlarge Rear view of the PET 2001.
Click to enlarge The PET 2001 had no fans installed. Everything inside was quite spacy, so there was no active cooling required. Note the hinge which connects the case cover to the back sheet metal.
Click to enlarge The PET 2001 is a totally integrated system. Even the data recorder (named 'datasette') was part of the control panel. With the consequence, that there was little space left for the keyboard.
Click to enlarge Just like the Apple II, the control panel is wegde-shaped. However here the similarities end.
Click to enlarge

The datasette used standard audio compact cassettes for storing and loading programs and data. The transport drive was controlled by software, whereas spooling (an loading/unloading a tape, of course) was a manual task.

The datasette is not able to position to certain records, it can just synchronize to a leading signal before the actual data starts.

For later models the datasette moved outside the system and was connected at the rear tape recorder port.

Click to enlarge

The PET keyboard is much like a calculator keyboard (in fact, Commodore just produced office machines including calculators before they started with the PET as their first computer). The keys are not displaced as it is for a typewriter keyboard, but rather placed in a rectangular grid. All but the space and enter keys are of the same size, and much smaller than a normal typewriter key. Fast typing was not possible with this keyboard called 'chicklet' keyboard.

Non-working keys in general can be revived by opening and cleaning the keyboard assembly.

Click to enlarge The Commodore logo isn't that spectacular. It was printed on a plastic sticker, which normally is the first PET item which gets lost.
Click to enlarge The power cord is fixed and makes up a unit with the fuse and the power switch at the back of the system. The PET 2001 can not be configured to 110V or 220V by the user, you had to specifiy the AC when ordering the computer.
Click to enlarge

The I/O connectors are a nice example for the overall design of the PET. Just openings in the rear sheet metal, with naked edge connectors from the mainboard looking through (this definitely would have been against honor at Hewlett-Packard). But at Commodore they thought it practically, and the I/O connectors are quite versatile.

From left to right:

  • Auxiliary audio tape/datasette connector (with motor control),
  • user port (with bidirectional 8 bit parallel plus serial, diagnostic and TTL video out signals) and
  • IEEE488-connector (for peripherals like floppy disk drives or measuring instruments).
Click to enlarge The PET's system bus with all address and data lines is provided by a connector called 'expansion port', which is placed at the lower right side of the system. The main purpose was to connect a memory expansion to this port, however it figured out that most memory expansions were internal solutions.
Click to enlarge The brightness control is a bit secluded at the back of the monitor. The monitor casing itself was part of the whole sheet metal casing and could not be separated from it.
Click to enlarge

One of the best ideas ever was to connect the upper (white sheet metal) part of the system with the lower (black sheet metal) part with some kind of piano hinge. So the whole system can be easily opened by just lifting the upper part of the casing much like an engine hood, and there is even a rod to fix the case in its position.

The system can be operated without any difficulties in this configuration, so it was highly serviceable.

Click to enlarge

The whole system mostly consists a single large mainboard, plus a second PCB for the analog display circuits. The electronics of the PSU (including the regulators with their heat sinks) are placed on the mainboard, just the transformer and the leveling capacitor are placed separated on the left.

Cables lead from the mainboard to the datasette, the keyboard and the display.

On this picture all ROMs have been removed from their sockets and replaced by a small piggy-back-solution for RAM and ROM by Nikolas Welte. The board shown is a second revision and already has DRAMs installed (the first models had static RAM).

Click to enlarge Detail view of the PSU part (transformer, capacitor and regulators).
Click to enlarge Upper part of the casing. Note the analog curcuit board for the display, the datasette and the keyboard assembly. All connected with cables to the mainboard.
Click to enlarge

Comparison of the Apple II and the PET. Note the totally different design. The PET comes with integrated monitor capable of 40 x 25 characters display, whereas the larger Apple monitor is an extra item capable of 80 x 24 characters.

The Apple II has no integrated mass storage storage, however it had been quite common to add Apple Disk II drives to the standard Apple, which are shown here.

The PET has a calculator keyboard with separated numeric block, the Apple II already has a (minimalistic) typewriter style keyboard.


A Comparison to the HP 9845A

HP 9845A

HP 9845A

Before we go into the comparison, let's state that that it can't be a 'which is best' approach, since already the entry price ($500 vs. $800 vs. $1,300 vs. $11,500) is totally different. If we just want to compare personal or home computers, we should rather do a comparison against HP's Series 80 (which arrived three years later), not against the 9845 series. More interesting is to compare the different design principles and technologies, which all go back to the know-how of the same year, and maybe see whether the result justifies the difference in invest.

But let's start with Apple. Steve Wozniak, the inventor of the Apple I & II and co-founder of Apple Inc., worked after his study as employee of Hewlett Packard, where he tried to convince Hewlett Packard's 9830 project managers to get into the home computer business. However, according to Wozniak, HP decided that a home computer would not fit into the product line of HP. When HP finally started an 8-bit computer project with the Series 80 (still $2,750 for the simplest configuration) and their own 8-bit processor (code name "Capricorn"), HP refused giving Wozniak the chance to participate in the development. So Wozniak finally decided to leave the company and concentrate on his work for Apple Inc.

The HP 9845A mainly had been the effort of HP to design a system with whatever seemed to be technically possible at the time. The product sheet as sketched by Jack Walden in fact had been some kind of an engineer's whish list, and initially wasn't really directed to a special target group or usage.

So, where now are the main differences to the "stars" of 1977?

System Architecture

Both the Apple II and the PET used the low-cost but easy to program 6502 microprocessor with a logic design much oriented towards the reference design for the 6502. Same applies to the TRS-80, which used the Z80 microprocessor architecture. HP in fact downscaled their minicomputers along with its main system architecture into the three-chip 16 bit hybrid processor for the HP 9845A and the other system components. And the HP as a true multiprocessor system used two of those processors in parallel for freeing the main logic from all the I/O work. Also there were two separate 16 bit wide bus systems on the HP (one as memory bus and another for I/O), whereas Apple, PET and TRS-80 used a single system bus design.

All that were indicators for being a real workstation. Another point was that HP did a highly complex approach with twice the amount of R/W memory, seven times the number of ICs compared to the Apple II, PET and TRS-80, and 33 PCBs again compared to one or two for the Apple, PET and TRS-80. Also the 9845A was capable of doing floating point maths in hardware, which wasn't even an option for the Apple, the PET or the TRS-80.


The system firmware of the HP 9845A was about ten times the ROM space used for the Apple, the PET, or the TRS-80, although all four used BASIC as the standard built-in programming language. The Apple II with its Integer BASIC was much a result of what Steven Wozniak thought would be useful for a programmer. It included a quite capable machine language monitor, but lacked string handling and any floating point maths. Later Apple licensed the common and more complete OEM BASIC developed by Microsoft and called it Applesoft BASIC. Commodore did this licensing from the very beginning and adopted MS BASIC for its PET system. Tandy started with a Level I BASIC derived from the public domain Tiny BASIC, but later licensed Microsoft BASIC for Level II. HP focused primarily on the ANSI BASIC standard, and did its own extensions (resulting in something which had not much in common with ANSI BASIC any more). Some at the time unique features were identifiers with up to 16 characters, code indentation, variables with local scope, matrix calculations, interrupt processing, full debugging and tracing capability, dynamic or static extensions of the already rich command set, and the possibilty to save the full system state on mass storage for resume after powering down. Plus the ability to enter commands and change data while a program is running.

The software base for the Apple II mostly was provided by third party vendors, who took profit from the open overall architecture of the Apple and created most of all many games for it. For the PET there was almost no commercial software available, most programs were developed by hobbyists and distributed via computer clubs and bulletin boards. TRS-80 boosted software sales just because of the high number of systems saled. Hewlett-Packard produced a large number of professional programs for any purpose on its own, but also supported third party vendors and even provided an exchange program for royalty-free software. Most HP software was distributed on cartridge tape, however there were also many plugable firmware extensions which were later integrated in more recent versions of HP BASIC.

There is one interesting aspect comparing especially the Apple II, whose Integer BASIC can - just as its name suggests - only do integer maths but no floating point, to HP BASIC, which can do floating point maths (even supported by the processor in hardware) but no dedicated integer calculations. HP BASIC simply makes no use of the integer capabilities of the hybrid processor but calculates all maths with floating point, even if a variable has been declared as integer. Finally this prevents utilizing the speed improvement for all cases where integer maths is adaquate, but maybe the speed improvements showed not to be that relevant.


Obviously the HP 9845A was much larger and heavier than the three other systems. The high complexity and the integrated printer needed lots of space, which also had to be actively cooled. The TRS-80 with its 2.1 kg (not counting the external transformer) was a fly weight. The Apple II with its 7 kg still was a real light weight compared to the HP 9845A with its almost 30 kg. Even the PET - although totally built on sheet metal - was still 5 to 6 kg less heavy than the HP.

Human Interfaces and Peripherals

If a special component was needed, both Apple and Commodore bought it off-the-shelf from the mass market. HP simply got it off its own shelfs or developed and produced it right for the 9845A on its own. Also, HP tried to optimize the qualitity of the system, especially for the display and the keyboard, with much experience from its own terminal production.

HP already provided a number of high-quality peripherals like graphics tablets, which were all supported by the HP 9845A from the very start. Later models also included a light pen and soft keys as standard especially for menu driven interactions.

Apple designed the Apple II with ports for up to three paddle game controllers (which also could be used for a single joystick) with the breakout game in mind. And it offered a full-stroke keyboard.

The PET had nothing comparable, however it was easy for the hobbyist to create his own solution by connecting a standard diskrete game controller with a simple adapter to the PET's user port. The tiny chicklet keyboard seems to be borrowed from desktop calculators of the time (actually Commodore had been in that business) and was inapplicable for rapid typing.

Same applies for the TRS-80, however the full-stroke keyboard was much better than the PET's chicklet keyboard, and there was the option to buy an Extension Interface unit, which offered connectivity to printers, additional cassette recorders and foppy drives.


A sad chapter for the HP 9845A. There was nothing more but a simple beeper with fixed duration, volume and pitch. The PET not even had a beeper, but with its VIA I/O wonder chip and the user port, producing sound with an external amplifier in fact was an easy task at least for the hobby electronics user. Same for the TRS-80, which had no on-board sound, but clever hobbyists programmed the cassette output to source an external amplifier.

With the Apple II, games were part of the concept (Wozniak created the breakout game for Atari before he started with the Apple I development, still with the game in mind). However from the hardware view, it was at the first glance worse compared to the HP. There was just a click sound which could be produced on the speaker by reading from a certain memory address. But again, the concept of the Apple design to avoid anything producing hardware costs and do whatever possible in software simply outplayed the HP: The software simply has to produce clicks in the right manner to provide sound with up to eventually four independent channels, where the sound circuit of the HP is totally diskoupled from any software control except generating always the same monotone beep.

As the Apple II was expandable by design, quickly a number of really sophisticated sound boards came up from third party side, like the popular Mocking Board. Those boards mostly utilize state-of-the-art programmable sound generators, freeing up the CPU for performing other tasks. For the TRS-80, the Orchestra board was available, which provided nearly unlimited stereo sound creation. It was plugged into the expansion port and used an extremely simple but versatile approach with a resistor based DAC solution. But all of those add-ons were optional and came from third parties.


The HP 9845A graphics option had four times the resolution of that of the Apple, the TRS-80 tried at least to mimic graphics with its block graphics characters, creating a 128 by 48 raster, and the PET lacked graphics at all (to be honest, the PET had a lot of graphical characters, which also could be used to produce some kind of pseudo-graphics with 80 x 50 'block pixels').

The Apple II used a simple but ingenious hack to produce color on an NTSC monitor, which therefore did not work for PAL/SECAM systems or RGB monitors (which weren't supported anyway). The HIRES graphics resolution is limited to 280 x 192 pixels. The graphics memory uses 8,192 bytes for a full screen, and each pixel is represented by just one single bit. That's common for monochrome graphics, but where is the color information? Now the Apple II has something like two palettes of four colors each (with white and black included in both palettes), which can be assigned with the most significant bit of every byte in the graphics buffer to the other seven bits which represent pixel on-off information.

Which color from a palette is finally shown in fact depends on the column of the pixel position (so actually the 280 pixels on a line are a bit of a fake concerning color, since color can't be assigned randomly to any pixel). This goes back to the fact that pixel clock in HIRES mode is exactly twice the NTSC color cycle, so that the position of the pixel determines the phase of the NTSC color signal. A perfect example for unorthodox but effective design.

Graphics are drawn by the processor simply by writing to the graphics memory. Some firmware routines written in machine language can be used to draw predefined vector images on the bitmap without destroying the background (a bit blitting technique called 'shapes' for the Apple II), which makes it possible to move animated objects over the screen.

Summarizing the graphics capabilities of the Apple, the PET and the TRS-80, those were very limited with respect to the quality of the display hardware (NTSC monitor for the Apple, TV set based hardware for the TRS-80). HP in turn had a strong interest in constructing a graphics display with outstanding quality. So HP's engineers kept a correct pixel aspect ratio, and built a high resolution (560 x 455 pixel) clear, crisp and steady display, but forgot to implement a direct access from the processor to the graphics memory (instead single pixel drawing commands were routed over the I/O bus) which finally limited drawing speed. So the display was used much more like a plotter and speed advantages of bit-mapped graphics were not fully available.

Later this concept was improved in that special graphics processors capable of vector generation were installed in the displays, which again made plotter like graphics lightning fast, but were still no replacement for bit blitting.


The most important difference of course was the target group, which finally decided on what influence the price has on the buy decision. The Apple II was made for the technically interested consumer, the PET for the technically skilled hobbyist, the TRS-80 for those somewhere in between who wanted to get the best compromise of features and price, and the HP 9845A was constructed from engineering professionals for engineering professionals. So the colorful and non-technical advertisements for the Apple in the BYTE magazine appealed to the consumer user interested in computers, the PET was sold more or less by its affordable price, over its role as computer system for schools and by the information moving around among hobbyists, and it is not quite clear where HP placed its nice but not outstanding advertisements outside its own publications (if ever).


All were designed as desk-top systems, but simply reflected different classes of computers. The Apple II, the PET 2001 and the TRS-80 – although quite different in their appearance – finally were home computers which had been designed mainly under cost aspects. Their origin was the hobbyist market which was started by the Altair 8800 and its clones like the IMSAI 8080. Altair and IMSAI were built around the early microprocessors and delivered the proof-of-concepts for technology, features and early use cases of personal computers, whereas the 9845 was a successor of the mini computers, built for highly professional environments and applications.

But there are also similarities. For instance the idea of everything important integrated in the system, and having it ready-to run just after unboxing. All systems were well serviceable (which had been a much higher challenge for the HP than for the much simpler Apple, PET or TRS-80). The HP and the PET both shared the concept to connect peripherals over IEEE488, which not even needed a special interface for the PET. Both the HP and the Apple already used modern and efficient switching power supplies (which has become standard on todays PCs).

As a conclusion, Apple, PET and TRS-80 represented perfect examples for personal computers, but the HP 9845A really was a workstation. And with the HP 9845A it was only the start which was continued in 1980 with the improved 9845B, the 9845C high end color system, and the high performance model 200 systems.