S100 Motherboard

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S-100 Motherboard
This S-100 backplane is an 9 slot active terminated motherboard for building and/or repairing small S-100 systems. It is primarily intended for bench-top work to repair and/or build boards without risking valuable legacy S-100 chassis.
The builder supplies their own power supplies, connectors, and other components.

This board is a copy of an 8 slot board (shown below) done by Andrew Lynch a number of years ago but with added features.
Added features:
    • 9 SLOT (One more slot added)
    • Active Termination (copied from v 03)
    • 1st SLOT is spaced 1” from other slots to accommodate IMSAI Style chassis. All other slots are spaced the usual 0.75”
    • Mounting holes for ALTAIR and IMSAI chassis. Hole size 0.138” (6-32 screws).
    • Two power connectors with different pin outs.
            o P1 with 0.2” (5.08mm) spacing. Hole Size 0.060”
            o P2 with 0.156” spacing. Hole Size 0.065”
    • Expansion Connectors with multiple function
            o Expansion IN/OUT
            o Test point
            o Active Termination
    • Jumpers to ground some S-100 pins as needed by IEEE-696.
           o Pins 20, 53 & 70. (Pin 53 on ALTAIR and IMSAI computers is needed for Front Panel operations and should not be grounded
             when using these front panels).
    • Thick board, approximately 0.100”.
    • S-100 connectors require 0.25” row spacing. Hole sized 0.040”.
    • Fused power.

First S-100 Motherboard

First S100 Motherboard-3

A new V4 version of this motherboard has been fabricated. Please contact Josh, AKA "Crusty" here to see if one is available.

Note that often S100 motherboards and boxes appear on eBay at very reasonable prices. These motherboards have "no moving parts" and are often a quicker/simpler solution to building an initial S100 bus system.
A good place to look is here.

A Custom 12 Slot S100 Motherboard with Active Termination.

I have found that 8 slots are somewhat limiting when using multiple S100 bus CPU's.
I describe here what is essentially an extension of Josh'es 8 slot motherboard.    It utilizes the same active termination circuit:-

Termination circuit

However I must point out that the placement of the parts is different from the above 8 slot board. I did this so the connectors could be placed further apart.
Also I should point out that the support holes to hold the board in place are different than in the above 8 slot board and because the actual boards are spaced further apart a new set of card guides need to be made (see below).

It took me 4 try's to get a noise free motherboard. Earlier two layer boards were not satisfactory. I ended up with a 4 layer board, connections on the top and bottom surface and two ground planes between all the connector pins for the middle two layers.

Board Size

There are a few minor modifications I made to the original 8 slot board.

For the Reset circuit:-

Reset Circuit 2

I found its best to really sink the reset line (S100 bus pin 75) to Ground. In my hands with the the resistor in Josh's circuit did not really go to zero with this Active Circuit.
So R18 (10 Ohm) should be replaced with a wire jumper.

The inline fuses (F1,F2 & F3) need to be capable of handling the high currents going through them. I used 3 AMP fuses for the +18V/-18V lines and at least a 10 Amp fuse for the 8 volt line.

One final modification:
This board is IEEE-696 compatible. For this the S100 lines 20, 53 & 70 are grounded. (This simplifies ground traces on S100 boards).

Building the Board.
This board has over 1200 solder joints so set aside a good block of time to do it carefully.
Most components are straight forward and common.
The LM4250 is meant to be an 8 pin dip. This is now a fairly old/rare part. I got mine on eBay. The "CAN" versions are more common.
You can bend the pins into two rows of 4 and solder the unit into the socket IC2. Then solder the socket into the board.

LM4250B

Examine the datasheet to identify pin1 from the LM4250 datasheet. It's important this unit is not too high or it will interfere with the S100 board above it.

Install all the components on the board. The TIP29 and TIP30 transistors do not need heat sinks. Make sure you solder in the 270 OHM resistor packs in the correct orientation.
Add the three LEDs D1,D2 and D3 with the long lead in the square pads.
The four power connections are screws as shown here:-

Screws

It is very important that the 8V, 16V and -16V screws don't contact the ground plane at the back and front of the board. Make sure the bolt heads at the back just cover the pads. This requires flat angled head bolts as shown above. Because these pads carry a significant current (particularly the 8V line), they should be soldered into place as shown above.
On the top side to be safe I add a plastic washer before adding the connecting wire and nut.

Power Connections

Power Connections

Z

Before adding the S100 bus connectors temporally hook 8 volts to the +8V nut. Carefully adjust the 2000 Ohm Pot R1 so pin 1 of JP10 is 2.3V.
Then install all jumpers on the board, JP10 included. Check each pin is 2.3V (except 20, 53 70, 1, 51, 2, 52, 50 and 100).

The S100 connectors are expensive so it is necessary to allot a good block of time to solder them in correctly.
The pads are very close on this board so be careful to use as little solder as possible.

Just to be safe I like to test the whole board with just 3 connectors first, (Slots 1,6 & 12). Hookup the power again and check all connector pins for 2.3V.
To test the board I use the Propeller Board, the 4MG RAM board and the Z80 Master CPU board. The Z80 monitor must boot every time.
The three power LEDs should light up.
If all is OK, add in the remaining connectors. When done, again check for 2.3V on the connector pins.

The connectors I used (see below), had extra long solder pins which protruded on the bottom of the board. I carefully clipped each pin down to the solder joint.
Here is a picture:-

Clipped Solder Pins

Next we need to prepare the board support. Its critical none of the bottom protruding pins touch the floor of the computers case.
To be safe I have a layer of clear plastic film on the box case.
The board is supported with 9 mounting bolts. Each bolt has a thick plastic nut/washer so the whole motherboard is raised away from the case floor.

Next we need to mount the motherboard in the computer box.
Carefully insert support screws as shown below.
Each screw is tightened into place with a nut and on top is placed a plastic washer. This insures no traces on the board contact the screws/case.
See below.

Box Supports

Next insert the motherboard in the case and tighten it down. Hookup the four power supply lines and again check some lines for 2.5V.

If you are using an external push button reset switch (besides with SW1) you need to hookup one of the switch's two wires to P12 pin 1.   The other switch wire goes to ground.
Pressing this external switch should zero the S100 bus line 75.

All that now needs to be done is add the card guides.   There are various ways you can do this depending on your case configuration.

card Guides

I have used two Plexiglas supports with holes to accommodate each card guide. It's tricky to get the card guide drill holes right. It took me three try's!
The height of the card guide has to be the exact height of an S100 bus card in a slot. This way board card ejectors work correctly.

Again initially for testing use as few boards as possible. I started with The Propeller Console IO Board, the 4MB RAM board and the Z80 CPU board.
The monitor worked correctly. Adding other boards I could boot CPM3 etc.

However there were problems using this motherboard with CPU boards that utilize 74VLCxxx bus buffers.
It would not work reliably with our FPGA Z80 CPU Board.

I looked at Godbout and Murrow Motherboards to see how they differ from mine.

I use 4 layers, two ground planes between the signals on the top and bottom.
They use two layers but run a ground line on the bottom between each S100 bus signal (see picture below).
As best I can tell the Godbout board does this only on the bottom layer. The Morrow board does this on both sides.

Murrow Motherboard

The Morrow board has termination set at 2.3V. The Godbout is at 2.6V. I tried both.

I'm surprised my 4 layer board with two ground planes din not act the same. I guess side by side signals are the problem.

I gave the design one more try by hand drawing ground planes between most S100 bus pins.
Here is a picture of that layout:-

Motherboard 4

Again while it worked fine with most S100 boards it failed with the FPGA Z80 CPU board. Better than before, but still not 100% reliable.
I Will terminate this project for now.

PARTS
Getting 100 pin connectors is a challenge these days. I got mine from eBay. You can do a web search for 100 pin edge connectors.
Newark has them at $17 each! (Part Number 85H7582) .

The card guides also are tricky to find. I use Mouser ones. (Part Number 749-SSD-400).
I got the LM4250 from Quest Components. It was the CAN type. I got an 8 pin DIP version from eBay.
All there other components are fairly common.

The links below will contain the most recent schematic of this board.
Note, it may change over time and some IC part or pin numbers may not correlate exactly with the text in the article above.

MOST CURRENT MOTHERBOARD SCHEMATIC (for the 8 slot board) (V4 3/8/2017)
MOST CURRENT MOTHERBOARD MANUAL       (for the 8 slot board) (V4 3/8/2017)
MOST CURRENT PDF LAYOUT OF THIS MOTHERBOARD   (for the 8 slot board)   (V4 3/8/2017)
Most current KiCAD files for this motherboard         (for the 8 slot board)         (V4 3/8/2017)

Other pages describing my S-100 hardware and software.
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This page was last modified on 09/29/2025