The Commodore PET, short for "Personal Electronic Transactor", was Commodore's first computer (not counting the KIM-1), introduced in 1976, beginning their evolution from a calculator company. It was originally designed by Chuck Peddle of MOS Technologies, which Commodore bought and turned into their semiconductor group. It was available in the late 1970's, and came in two models, a 4K model originally selling for about $495, and a whopping 8K model running about $795. It was on the scene around the time of the TRS-80 and Apple II. The 4K model was discontinued in April 1978, and the RAM increased even more with later PET models.

The machine had everything it needed in a single unit. The large case was sheet metal for most models, plastic for a few, with a "space-age" look reminiscent of 60's and 70's science fiction movies. The monitor was on the top of the case, and the front had a keyboard and keypad on the center and right side of the front wedge, and a cassette deck on the left side of the earlier models. The first version of the monitor was a 9" 40x25 line black and white one, and the later CBM-80 model had a 12" 80x25 green monochrome screen.

The single unit was one of the selling points of the PET. There was only one cable to connect, the power cable, and it was one of the first machines to include its own monitor. It also made it possible to consider the machine as a "portable", though it was a far cry from modern laptop computers, weighing in at 44 lbs.

The original version of the keyboard used a rubber "chicklet" style, which was later replaced by a more standard variant due to supply problems. The chicklet keyboard was undersized, making touch-typing difficult, and the membrane keys had silkscreened tops, making them prone to being worn away, leaving a bunch of blank keys. When Commodore got rid of this keyboard, the cassette deck had to be removed to make room for the larger one.

It was powered by a 2 MHz 6502 microprocessor, top of the line for the day, and contained 14K of ROM which offered built-in BASIC and a small operating system. There were four ports on the machine, an IEEE-488 port, an 8-bit parallel port, a port for a second cassette recorder, and a port to connect to the system bus. The bus port was later used to connect the optional 5 1/4" disk drive. The bus did not use S-100, which at the time would have allowed some add-on cards to be used. The RAM was originally 512x4 bit 6550 chips, later replaced with the newer 2112 512x4 bit chips.

The internal basic, from Microsoft, is similar to MITS 8800 BASIC, offering string commands such as LEFT$, RIGHT$, and MID$, along with cursor-controlled editing. It could handle integer and floating-point math, with 10-digit accuracy. Variables could be named with full words, but only the first two characters were recognized by the interpreter. Graphics were limited to a set of 64 defined characters, part of the PETSCII system used instead of ASCII, though a special mode would replace them with lower-case letters.

Later, an upgraded set of ROMs was available for the PET, shipping in many of the models. It had the IEEE-488 disk routines implemented, which the original did not have. A machine language monitor was included, and large number of bugs were fixed, including one that limited arrays to 256 elements. Later, a 4.0 version of the ROMs was released, with better disk commands, and added screen formatting commands. The 4.0 ROMs are required for handling the 80x25 character screen.

The Killer Poke

One of the more famous "bugs" in the PET was The Killer Poke. This is one of the few machines where commands entered in software can damage hardware.

Original PETs, with the 9" screen, were very slow when printing characters. The print character ROM routine waited for the interval between screen scans before writing characters, this was done to reduce RAM conflicts, and prevented snow. This was controlled by an I/O chip.

Eventually, people learned about this, and figured out a way to speed it up. By doing the right POKE command (writing to memory) at location 59458, the video controller would update much quicker. Even a few magazines eventually listed this POKE - it was like a free upgrade.

When the 12" model PET was introduced, Commodore had upgraded the display controller. It displayed much faster, and this POKE was no longer necessary. However, with the adjustments for the new screen, the POKE pointed to a different area in memory. Now that area of memory affected one of the newer screen capabilities, and if left running, will damage the video circuitry.

When a program with this POKE command is run, as the characters display, the screen first begins to warp, and then the display stops, and the machine becomes unresponsive. The only thing to do at this point is to turn off the machine, and do it as soon as possible!

Model Information:

  • PET 2001 Series

    Had the steel case, 40x25 monitor, came in both 4K and 8K models. Used the chicklet calculator keyboard.

  • PET 2001 8N/16N/32N Series

    Introduced later, the number before the N indicating the amount of RAM. The keyboard was the larger sized version. Came in both steel and plastica cases, and the monitors, while still 9" 40x25, shipped with both black and white and green monochome monitors. LAter versions had upgraded internal ROMs.

  • PET 2001 xB

    This model was labeled with "CBM", or "Commodore Business Machines". Had the full size keyboard, the upgraded ROMs, and most were made with the plastic cases. Still used the 40x25 monitor.

  • PET/CBM 40xx Series

    The xx represented the amount of RAM on the machine. More or less identical to the PET 2001 xB model, though later version started shipping with the 12" 80x25 monitor, along with an internal sound chip and speaker.

  • CBM 80xx Series

    80x25 12" display, and used the business variant of the keyboard - does not show the graphics symbols on the keytops. Had upgraded ROMs, and eventually was released in 64 and 96k versions.

  • PET SP9000 SuperPET (aka Micro Mainframe)

    80x25 monitor, 96K of RAM, multiple character sets, could load other programminbg languages off of disk, a 6809 coprocessor board, and a RS-232 interface.

Sources:
The Obsolete Computer Museum - Commodore PET, http://www.obsoletecomputermuseum.org/pet.html
Commodore PET Hardware Review, http://www.trailingedge.com/~dlw/comp/petrv.html
Commodore PET FAQ, http://www.6502.org/users/andre/petindex/local/petfaq.html

When my dad first dragged home a PET from work, I thought I'd died and gone to heaven. That thing looked like the future! It was white, with a little built-in CRT and probably made out of folded sheet metal (it weighed about a hundred pounds). I had some friends with TRS-80s and this thing made them look totally punk-ass. I mean, a TRS-80 just looked like a keyboard stuck to a TV set, while the PET looked like a NORAD/HAL-9000 love child. I played ASCII games -- Pac Man, Space Invaders. It had 8K and a cassette drive.

My dad was doing a software project for it and had some appropriately uber-geek high school students doing some of the programming (first geek exposure, very formative). They let me hang out while they were working, listening to Styx and Loverboy while the lines of code churned out. Once, when a dupe of some software had to be made, Rick (geek 1) stuck the cassette in his tape-to-tape recorder and turned on the sound. I'll always remember that squeal of bits coming through the speakers.

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