The HUH Electronics Model 8100

written by Matthew Reed

Advertisement for HUH Electronics

HUH Electronics advertisement from the March 1979 issue of Intelligent Machines Journal

After the Radio Shack TRS-80 Model I was introduced in 1977, many companies were said to be developing an interface to allow the Model I to use the wide selection of S-100 hardware available at the time. HUH Electronics of San Mateo, California was one of the few companies (if not the only) to actually release a S-100 interface. Their Model 8100, introduced in November 1978, had six slots to allow up to six S-100 devices to be connected to the Model I.

The S-100 Bus

The S-100 bus standard was created by MITS in 1975 for their Altair 8800 computer. The Altair 8800 was designed with one mainboard with five 100-pin edge connectors. These connectors were used to attach the CPU, memory, and other peripherals to the Altair. These connectors followed a standard that became known as the “Altair bus."1 Many people started creating and selling hardware designed to plug into the Altair bus. Competing computers, such as the IMSAI 8800, also used the standard, but under the name “S-100 bus.”

Despite some problems, the S-100 bus was flexible enough to act as a connection standard for the rapidly expanding computer market. It even provided interrupt and direct memory access lines not used by the Altair 8800, but available for use by later computers.

Due to its popularity, S-100 bus support was considered for the original design of the TRS-80 Model I. It was rejected as being too expensive to implement in a low-cost computer. Instead, the Model I offered a proprietary expansion port that could be used to attach external hardware such as the Radio Shack Expansion Interface.


HUH Electronics sold three versions of the Model 8100.

  • The original 8100 provided six S-100 sockets. The base version cost $185.00 for a kit and $245.00 fully assembled. Available options included sockets for 16K of memory and an I/O package featuring one serial and one parallel port. A fully equipped 8100 cost $295.00 for a kit and $375.00 fully assembled.
  • The Mini-8100 provided four S-100 sockets. It cost $115.00 for a kit and $155.00 fully assembled. No options were available for the Mini-8100.
  • The Mini-8100S had no sockets but was designed to plug into an existing S-100 mainframe (after removing the CPU card). The Mini-8100S cost $95.00 for a kit and $125.00 fully assembled.

None of the 8100 models came with a power supply. Oddly (at least by modern standards), none of the models came with any kind of enclosure.

HUH Electronics also sold a S-100 interface for the Commodore PET called the S-100 MPA.


The HUH Electronics models had good compatibility with S-100 hardware. For example, a review in 80 U.S. Journal noted that the MicroSounder from American Micro Products and the Percom CI-812 worked without problems. There were a few exceptions:

  • S-100 hardware couldn’t use direct memory access because the Model I didn’t support it.
  • Due to the dynamic memory in the Model I, S-100 hardware couldn’t halt the Z80 for longer than 1 millisecond.
  • S-100 pin 26 (pHLDA) wasn’t supported by the 8100 and couldn’t be used.
  • Any S-100 hardware that required the clock on S-100 pin 49 be precisely 2 MHz wouldn’t work. The 8100 provided the Model I 1.77 MHz clock on that pin.

The biggest compatibility problem was the lack of direct memory access. This ruled out some popular hardware such as the Dazzler from Cromemco.

The HUH Electronics Model 8100 was probably the most direct target of the World Power Systems fraud. World Power Systems ran advertisements in 1979 listing the “TRS-80 to S-100 Bus Cable Adapter” (equivalent to the Mini-8100S) for $119.95 assembled. Their equally non-existent “TRS-80 to S-100 Bus Model RSB-A” (equivalent to the 8100) cost $289.95 assembled and came with a case and power supply (unlike the 8100 which came with neither.) According to the advertisement, the Model RSB-A generated S-100 signals “fully compatible with the TRS-80."2

HUH Electronics and its products, including the Model 8100, was purchased by California Computer Systems in early 1979. California Computer Systems continued selling the Model 8100 until they discontinued all of their TRS-80 products in late 1980.

  1. In 1983, the S-100 bus became an actual standard, IEEE Standard 696. ↩︎

  2. This would have been very difficult because the Model I didn’t support direct memory access. ↩︎

Categories: Hardware