Case Study - Restoring an Intel System 80/10

A horizontal ribbon.


This is the story of my work to bring an Intel System 80/10 up and running under the CP/M-80 Operating System on 5.25-inch FDDs. This machine consists of three multibus boards: iSBC 80/10 CPU, iSBC 064 64K RAM and iSBC 208 DSDD FDC.

The Hardware Reference Manual (HRF) for the System 80/10 I have is Intel Order Number 9800316B.
The Hardware Reference Manual (HRF) for the iSBC 064 I have is Intel Order Number 9800488B.
The Hardware Reference Manual (HRF) for the iSBC 208 I have is Intel Order Number 143078-001.

To do this I had:
1) An unknown iSBC 80/10 CPU (Picture).
2) An unknown iSBC 064 64K RAM (Picture).
3) An unknown iSBC 208 DSDD FDC (Picture).
4) A unknown 7-slot multibus card cage.
5) An known good ATX Power Supply.
6) A known good 5.25-inch dual FDD 360K drive set.
7) The manuals listed above.
8) Two unknown 2708 EPROMs on the iSBC 80/10.
9) Known good bootable CP/M-80 media.

I had no idea of the condition of any of these boards or the card cage. I needed to test them all.

Card Cage and Power Supply

I needed to get a card cage powered, add a power switch, a reset switch, and a power LED on it. I modified a 3-slot cage with the power supply wires soldered directly to the card cage backplane. This is shown here. After some rethinking I modified it to be modular using a Molex connector to replace the ATX connector. Later I modified a 7-slot cage as shown here. I now had two card cages and a power supply that worked together. All voltages checked out correct on the multibus connectors.

I have rethought the replacement of the ATX power connector. I now have a supply of ATX power sockets to make this connection, thus not having to modify the ATX power supply. I need to get some protoboard with a hole grid at 0.156 centers to use to mount the ATX power connectors to the card cage. The pins on the ATX sockets are very weak!

Power supply was good!
Both multibus card cages were good!

iSBC 80/10 CPU Board [1975]

I used the original Monitor ROMs on the iSBC 80/10 to test it. It gave me a "." prompt. The source code for this Monitor is here. I had to work through the "standard" RS-232 configurations to get connectivity to my desktop via a terminal emulator program.


Cross Development Tools

I needed to be able to do 8080/8085 cross development on my Windows 7 machine. These cross-assemblers and other tools are discussed here. I used these tools to assemble and build ROM images of Intel Monitors and develop my own Monitors for the iSBC 80/10 CPU board.

Monitor Mon80v40 Software

I did not have the capability to read or write the 2708 EPROMS. These require many voltages and are not worth the effort. I wanted to be able to run a Monitor in one 2732 EPROM. I modified the iSBC 80/10 remove the odd 2708 voltages from A23 and bring up the two additional address lines from A42, the decoder for the ROM CS lines. This makes socket A23 a 2732 capable socket.


I had to port my Monitor program to the iSBC 80/10. I had last used this Monitor on some 8085-based single board computers I had built. The modifications that became my Mon80v40 are here. I included a RAM test routine stolen from the H8, and Intel Hex load and store routines.

New Monitor

Using the Mon80V40 versions, I was in an edit, assemble, and test loop on the iSBC 80/10 board. Once I had the code stable I was able to validate the "A" version of the Monitor.

In order to run CPM-80 on this system I needed a way to disable or shadow the onboard ROM and RAM on the iSBC 80/10. I chose to use a couple of bits from one of the I8255 chips on the iSBC 80/10. I added a 74LS32 on top of A44 to disable the address decode for the RAM and ROM. The black, brown, and red wires were for an earlier EPROM adapter. I needed to put the CPU in the bottom slot of the card cage so I needed a low-profile solution. Hence the wire changes on the iSBC 80/10.

As an after thought I believe I will modify the RAM/ROM disable to one bit from the 8255. I always turn both at the same time, anyway.

DSC_0012 DSC_0015 DSC_0017

Here is a diagram of the modification:

RAM-ROM Disable

Version "A" of Mon80v40 was good!
iSBC 80/10 CPU board was good!

iSBC 064 RAM Board [1977]

I used the Version "A" and "B" Monitor ROM to verify the iSBC 064 RAM board had good memory. Not bad for a board over 37 years old!

iSBC 064 RAM board was good!
Version "B" of Mon80v40 was good!
The ROM and RAM disable were good!

iSBC 208 Floppy Disk Controller Board [1978]

The iSBC 208 was the last computer board to be tested. I configured it for DSDD 5.25-inch disks and placed it in the card cage. I wrote some simple read, write, seek and home program to exercise the iSBC 208 directly with the DSDD 5.25-inch disk pair.

iSBC 208 FDC board was good!
The disk subsystem was good!

iSBC 208 Driver Software

I had source code for the driver for the iSBC 208 FDC board. I modified and integrated this driver into version "C" and "D" of my Mon80V40 Monitor. I had to modify the interrupt jumpers on the backplane so it would work properly with interrupts. I wrote some simple read, write, seek and home program to exercise the iSBC 208 through the driver software with the DSDD 5.25-inch disk pair.

iSBC 208 driver software was good!

Operating Systems

These early systems were designed to be used to develop hardware for control applications. They did not come with any operating systems. They were sold with small backplanes and an Intel blank multibus prototype board. The Intel MDS systems were used to develop software and validate the operation of the new board. Disk drives were very expensive. Some MDS put all the development software into ROM to eliminate the Operating System! Intel also offered the tools on paper tape.

Now I need to port an Operating System (OS) to this hardware.

CP/M-80 Operating System

I was most familiar with CP/M-80 Version 2.2 and had complete source code with some modifications I liked. I chose to port this OS to the Intel System 80/10 first. I don't believe this OS ever ran on a System 80/10. The porting of CP/M-80 is discussed here.

CPM-80 boots on the Intel System 80/10! This project is complete!

A horizontal ribbon.

Last updated: 19 July 2015