STEbus 486DX PC AT Compatible Board

• Make VGA cable, check video output
• Make PS/2 to PC AT keyboard translator
• Download Windows 98/NT/2000 installation CD with licence number
• Install Windows on an IDE drive

Booting from an M-Systems Disk-On-Chip device looks intriguing, because it fits in a DIP32 memory socket.

It occupies 16K memory space, of which half is paged memory and half is control and paging registers. 

They are not cheap - eBay has them for between £19 (32MB) and £24 (65MB). Typical SD cards give a thousand times more for the same money.

There is also the nuisance of programming it. I'm not familiar with writing disk drivers and I doubt I would enjoy it. M-Systems refer to their source code but I have not found a copy of it yet.

Lots of documentation here:

https://manualzz.com/doc/7071764/user-manual--diskonchip-software-utilities-for-trueffs-6.x

My STEbus rack already has a 3.5" floppy drive, and spare power connectors for hard drives.

Amazingly, I found I still had a set of 3.5" floppies with MSDOS 5 and Windows for Workgroups v 3.11 so that's a start. I have a 3.5" floppy drive and a spare 80GB 2.5" IDE drive. Plus a 3.5" to 2.5" adaptor.

2022-01-17

Applied 5V power. Nothing happened. Measured output of 5V PSU - nothing. I don't know why, though it had been plugged in for ages without drawing current. Disassembled, the fuse is intact.

Replaced PSU with one with 5V output. Measured 5V on bus rails but the fan and LED were off. 5V is getting to the fan, but no rotation. 

Replaced board with second board that I bought. Fan spins. LED D5 does not light, but this is a user LED - it does not indicate any system activity like drive access.

I conclude the first board fan is dead. Fan type is Sunon KDE0502PFB1-8, which is 25 x 25 x 10 mm at 0.9W. Nearest Digikey equivalent is 02510SS-05P-AT-00 for £8.15 or Sunon KDE0502PFB2-8VA.

2022-01-18

Bought a pair of replacement fans, and a PS/2 Y-cable, which arrived the next day.

The fan connectors differ from the original Bergstick style connector, so some modification is needed. No hurry, I shall use the board with the working fan for now.

2022-01-19

I removed the dead fan so I could fit the new fan. Just checking, I applied 5V to the dead fan and it worked! The fan is fixed to the heatsink with some thin sticky foam rubber shaped like a square with a round hole in the middle. I suspect some of it had squashed into the middle, stopping the fan blades from turning. Some fine knife work should fix that.

The 50-way cable now connects to a female DB9 connector.

Ordered various bits for board cabling.

The board does not have the standard D-type connectors for the keyboard, mouse, COM, LPT and VGA ports. There simply isn't room on the 100mm edge.

A 50-way header has the COM and LPT ports, which can go to IDC D-type sockets. I'd rather it had been one 26-way header and two 10-way headers, as these are the usual headers used and it is easy to find ready-wired cables.

I will need to make up a cable for the COM and LPT ports, but that should not be hard. I already have a 50-way to LPT cable that I made for a previous project, but I will have to order and fit two COM sockets.

The VGA header has 10 pins, there are no I2C signals nor any standard pinout. The nearest one I have seen is a 16-pin header with IDC cable wire 1...15  going to VGA pin 1...15, with wire 16 unused.

There is a 14-pin header carrying signals for the mouse, keyboard, speaker and battery rail. I bought a Y-cable splitter which has the mouse and keyboard signals on separate pairs of pins:

https://old.pinouts.ru/InputCables/Ps2KeyboardYThinkpad.shtml

https://en.wikipedia.org/wiki/PS/2_port#Port_availability

Normally pins 1 and 5 are used for both keyboard and mouse, but pins 2 and 6 carry the mouse signals in a combined cable.

Needed per board:

50-way IDC socket + 50-way ribbon cable.
26-way IDC socket  + 25-way IDC DB25 connector for LPT port.
14-way IDC socket for keyboard, mouse, speaker, etc.
10-way IDC socket + 15-way Mini DB15 connector, for VGA .
10-way IDC socket + 9-way IDC DB9 connector, two of each, for COM ports.