When Commodore introduced its PET line of personal computers in 1977, they needed a bus to connect peripherals. Rather than do something proprietary, they chose an industry standard, the IEEE-488 bus originally designed by HP. It suited Commodore’s needs at the time, even though it’s pretty obscure today.
The IEEE-488 bus
IEEE-488 is an 8-bit bus that transfers a full byte of data at once in parallel. It generally uses a Centronics-style 24-pin D-sub connector. Each connector is double-sided, so you can chain off one peripheral to connect the next one. You can connect up to 31 devices on a single bus. Conceptually, it may remind you a little of SCSI.
In theory, the IEEE-488 bus could transmit up to one megabyte of data per second. In practice, that was much faster than any PET peripheral could manage in the 1977-1982 timeframe.
IEEE-488 as an industry standard
HP designed IEEE-488 in the 1960s as a bus to interconnect test equipment. HP later repurposed it as a computer bus, and licensed it to other manufacturers. It became a de facto standard, which HP called GPIB, or General Purpose Interface Bus. In 1975, the IEEE formalized the standard and assigned number 488 to it, making it an industry standard. Commodore used it for connecting disk drives and printers to its computers. In Europe, IEEE-488 was known as IEC-625, sometimes simply called the IEC bus.
The standard continued to evolve after Commodore stopped using it. The early IEEE-488 standards formalized the mechanical connectors, electrical characteristics, and the basic protocol parameters, but there was no standard for the commands or the data. This kept you from plugging a Commodore disk drive into an HP computer that used the same bus. It wouldn’t hurt anything, but the two devices wouldn’t communicate.
Standards for the commands and protocols followed in the late 1980s. While it’s still used for test equipment today, it’s very niche. It was long ago supplanted as a computer bus by other, more versatile standards. When someone wandered into Best Buy in 1995 with one of these cables, it confused all of us.
Why Commodore stopped using IEEE-488
The PET series sold well in some parts of the world but was too expensive to gain mass adoption. In 1980, Commodore decided to try something different, and rather than chase the business and educational markets, to experiment with an inexpensive home computer. The result of this was the VIC-20, a $299 computer that plugged into a television.
The IEEE bus wasn’t terribly expensive, but it was too much for Commodore to hit that $299 price point. Commodore didn’t expect to sell a lot of disk drives; they figured most of their users would use tape instead, so the IEEE bus was just going to be a waste of money and capability. So instead, Commodore simplified the IEEE-488 bus, cutting it down to a serial bus that transmitted one bit of data at a time instead of a full byte. It was a lot slower, but it required cheaper electronics inside the computer and peripherals, and it allowed them to use a much smaller, cheaper round DIN connector and cables, similar to what MIDI uses.
For some reason, most people refer to this serial IEEE-488 implementation as IEC, but that’s a bit of a misnomer. Commodore’s serial bus is no more IEC-625 than it is IEEE-488. In the 80s and 90s, everyone I knew just called it serial. But in the Internet age, we call it IEC.
The Commodore 64 certainly could have benefited from the faster disk drives of the PET, but it also had serious price considerations. Hitting a $595 price point was difficult due to the cost of memory, and Commodore wanted it to use the same peripherals as the VIC. So it received the same serial IEC port. Arguably Commodore could have revisited later when prices came down, but continued forward and backward compatibility prevented that from happening. Fast load cartridges became very popular to compensate.
IEEE-488 cartridges for the Commodore 64 and VIC-20
Commodore and other companies did sell interface cartridges to add a full parallel IEEE-488 bus to the C-64 and VIC-20 so they could use the bigger, faster PET disk drives. They weren’t super popular, as these disk drives were uncommon and expensive. But they did see use with BBS operators, where the storage capacity facilitated file transfers and large message boards.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.