Cromemco and the fate of the S-100 Computer

The 1981 Cromemco System One – 64K RAM Z-80 CPU, CDOS Operating system 5 1/4″ disks. (The leather keychain is not part of the system)

A computer like the Cromemco System One used a microcomputer architecture platform known as “S-100 bus” The “100” in S-100 refers to the number of electrical contact points available for each component installed into a passive backplane. The interconnection of backplane wires or the “bus” in an S-100 system was fitted with a series of 100-pin card-edged slots that allow “S-100 card” control boards to be inserted in sequence. The “S-100 computer” was thus a series of S-100 cards installed into the S-100 backplane wired to the S-100 bus standard. S-100 cards are often separated by function. A well-equipped S-100 computer system might consist of a CPU card, front panel card, RAM card, printer card, serial card, modem card, disk drive card, and a display card. Any card on the S-100 bus, especially the microprocessor card, could communicate with any other card installed on the same S-100 backplane.

How did S-100 become a standard?

It all started in 1975 with MITS’ early Intel 8080 microprocessor computer, the Altair 8800. The Altair was a tremendous hit and MITS quickly found that it could not keep up with orders. Electronics and component startups (such as Cromemco) capitalized on MITS’ inability to meet demand by producing and selling their own MITS Altair-compatible “clone” controller cards, and eventually entire “Altair clones”, to compete with MITS directly (i.e IMSAI 8080). The tremendous demand for the Altair and the fact that other manufacturers chose to follow MITS’ lead resulted in the S-100 bus becoming the leading platform for small business micro systems. A few years later the US-based IEEE published a refined S-100 bus (“IEEE 696“) standard based on the original MITS version, with a few improvements.

S-100 was the first popular microcomputer bus because it allowed one to choose and install “S-100 standard” CPU, RAM, printer, serial, modem and display controller cards to suit the needs of the user. Dozens of manufacturers made standardized S-100 computers and peripheral cards at one point or another. Like today, one could buy a name-brand core system and then fill it with clone components to save money.

The default operating system for S-100 business computers was Digital Research’s CP/M, but there were many other disk operating systems available at the time. For example, Cromemco’s systems came with CDOS written by Richard Roth and Ed Hall.

Cromemco was one of the first Altair cloners and they produced their own well-built, “S-100 bus” business computers and components. These sold well. Typically their machines featured Zilog Z80 microprocessors which were faster than the Intel 8080, and more easily ran the CP/M disk operating system.

By the early 1980’s Cromemco was the top-selling S-100 computer manufacturer, but the party came to an abrupt halt when the IBM PC (with its new “ISA” bus) appeared. Sales of all S-100 systems including Cromemco’s machines almost immediately faded. Cromemco attempted to counter IBM’s PC with a 16-bit Motorola 68000 CPU microprocessor S-100 computer capable of running the CROMIX operating system, but these computers did not sell. By the mid-late 1980s Cromemco discontinued S-100 production and the company went on to produce other hardware.

The Cromemco 3102 Terminal

What did it look like to operate a typical S-100 system? Meet the 3102 terminal (below), a re-branded (“OEM”) Beehive terminal configured for use with Cromemco’s System One and Z2-H (System Two) computer models. No color, no graphics, just text.

The 1980 Cromemco 3102 Computer Terminal
Screen display of the boot sequence (ROM v. 02.52, CDOS 2.53 disk operating system) and the command to set the system time.

Click here to view clearer photos of the status bar at the bottom of the above image. 1 2 3 4

To generate the above output one must complete the following:

  • Start the 3102 terminal in LOCAL mode and press ESC –  (escape, minus) to show the status line
  • From the RDOS prompt, boot into CDOS (from disk in the computer)
  • At the CDOS prompt enter the command: stat/t
  • Enter the time when prompted.
Follow up to the above screenshot, after re-running the CDOS STAT command without /t

More Photos of the System One computer More photos of this 3102 terminal