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Cromemco System One

Cromemco System One

The Cromemco System one is a 68010 based Unix system from the early eighties. Cromemco had previously manufactured Z80 based system around the S100 bus.


The System One chassi is a rather small S100 bus chassis with 8 slots, a full height 5.25 inch floppy drive,  a full height hard drive and a PSU.

Chassis top
The floppy drive is a Tandon TM100-2A and the hard drive is a 15 Mbyte (6 heads 306 cylinders) IMI 5021H. It looks like someone has modified the power cabling from the power supply to the backplane since the wires has been soldered directly onto the the connector. Then there is some kind of problem with the key switch. Sometimes it will not through connect in the on position. Has it been changed? There are some evidence that it has been tampered with.

S100 bus

S100 bus backplane

As the name implies the connector has 100 pins. The S-100 bus was a passive backplane bus first used for the Altair 8800 computer. The signals is mostly the signals of the 8080 microprocessor. It was later standardized as IEEE 696-1983.

This system

Backplane with cards
This small system has seven out of eight slots populated with boards. The two top most cards, the CPU board and the first memory board was removed when the photo was taken.

The small orange and black wires that seemingly irrational connects between different boards is the interrupt priority chain. It should be an unbroken chain of cards attached to this cable, from the topmost priority board, the 64FDC to the lowest priority board. In this system however the chain broken since one connector in the middle is not connected! Has a card been removed? Slot number four from the top is empty so just maybe there were supposed to be a card there connected to tis interrupt chain. Likewise, the DMA chain from the STDC just hangs lose.

 1 XPU Main CPU board with PGA mount MC68010 CPU and Z80B CPU
 2 1024KZ 1 Mbyte memory board
 3 1024KZ 1 Mbyte memory board
 5 BIART Synchronous communication board
 6 OCTART Octal asynchronous communication channels board 
 7 STDC Controller board for the ST506 type hard drive
 8 64FDC Floppy drive controller and console serial port
The system seems to be manufactured around 1983-1984. Given the age it is quite impressive that it has 2 Mbyte of memory. A little bit more than normal at this time I would guess.


The Cromemco XPU board. It has a dual CPUs, a 10 MHz MC68010 and a 4 MHZ Z80A CPU. The top left 34 pin connector is used to connect this board to the XMM board which do demand paged memory management. The manual for the XPU card is here.
Cromemco CPU

The Cromemco 1024KZ memory board which holds one megabyte. Manual.

Cromemco 1024KZ

The Cromemco BIART board for synchronous serial communication. It uses a Z8530 SCC chip which provide two independent communication channels for protocols like HDLC. Manual

Cromemco BIART

The Cromemco OCTART board which provide 8 asynchronous serial channels by using for Signetics SCN2681 UART chips. Manual.

Cromemco OCTART

The Cromemco STDC board to interface ST506 type MFM hard disk drives. Manual.

Cromemco STDC

The Cromemco 64FDC card to connect to 8 inch and 5.25 inch floppy drives. Also includes the serial console port.

Cromemco 64FDC

Trying to Boot

So with the PSU checked and all cards reinserted power was applied. This was what I got when connecting the console serial port. 

Preparing to boot, ESC to abort

Cromemco RDOS 03.08

And then the drive motor of the floppy was running. As far as I understand this means that I need to have a floppy inserted to have it to boot and then continue on the hard disk. 

Drive imaging

Using David Gesswein's excellent mfm emulator I dumped the raw contents of the drive. Now this is a format which is very specific to the mfm emulator and contains the actual transitions of the read data. It has to be decoded to create something useful. If anyone find it interesting the raw transition data file is stored here