S-100 Bus 12 Slot Motherboard  
My current "Test S100 bus System" is now over 15 years old. Countless S100 boards have been pulled in and out of the system. The front S100 bus connector has been replaced two times. It is a George Morrow 12 slot "Wunderbus" Motherboard. Here is a picture of the setup:-
       
The board has on-board active termination and excellent ground plains to yield clean reliable S100 bus signals.   Unfortunately it is not a common motherboard and rarely appears on eBay.
I decided to construct and build my own.   This turned out to be more difficult (and expensive) than I expected. It took 4 prototypes to get everything right.  Early two layer designs done with KiCAD were just not reliable at high speeds. Four layer boards with two internal ground planes were better but failed with chips using 74VLCxxx bus buffers such as those used in our FPGA Z80 CPU board.
In the end I had to hand layout a four layer board with two internal ground planes and ground flood planes between the S100 bus traces on the top and bottom of the board as well as between the S100 bus connector pins.  I could not get KiCAD (V4.1) to do this! It took a full day of work.  Here is how the final board looks:-
        
The trick is to place small vias between the traces that connect to the inner two layer flood ground planes and let KiCAD then do a ground fill in the local area

Active Termination
Active termination of a bus is where we prevent buss signals running down the bus and bouncing back from the end creating noise on the bus. A number of approaches have bee used. For simple TTL logic the most effective way is to mention the bus line when not drive low or high at about 2.3v-2.6V.  Easily done with a pull-up and pull down resistor. However for 100 bus lines this is inefficient power wise and largely unchangeable.  A better solution is to use an adjustable OpAmp circuit to set the line voltage.  Here is the circuit we will use:-
      
By adjusting R1 which alters the input to the LM4250 OpAmp you can set the bus line between ~2.0V and  ~3.5V. Every (non-power) S100 bus line has a 1K pullup resistor. 

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 as shown here:-
      
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.

Building The Board
This board has over 1200 solder joints so set aside a good block of time to do it carefully. It took me a full day to do this board. Most components are straight forward and common. Please study the schematic before building the board. Install all the components on the board. The TIP29 and TIP30 transistors do not need heat sinks. Make sure you solder in the 1K OHM resistor packs in the correct orientation. I like to use 10 pin linear sockets for these in case you need to change the value of the resistor SIP. 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:-
    
These are standard wire clip connectors.  You can use screws if you like as shown above, however be very careful the metal of the screws does not touch 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.
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 the 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. The ground pins need more time to heat the pads on this board,
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:-  
    
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 S100 bus connector pins.

Computer Box Assembly
Next we need to prepare the board support. Its critical none of the bottom protruding S100 bus pins touch the floor of the computers case.  To be safe I have a layer of clear plastic film on the floor of 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.
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.
   
Next insert the motherboard in the case and tighten it down.  Hookup the four power supply lines and again check some lines for 2.6V.
     
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.
  
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.
     
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.
All my "regular" S100 boards work fine with this motherboard. However I found that in order to run it with boards that utilize 74LVCxxx buffers (such as the FPGA Z80 CPU board) it was necessary to replace the 270 Ohm network resistors pulling up each S100 bus line in the classical Compupro motherboards with a 1K resistor network.  This appears to be due to the differences in the current sinking differences between 74LSxxx and 784LVCxxx chips.

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.
A TIP29C or TIP32C NPN transistor can be used instead of the older TIP29
A TIP30C or TIP32C PNP transistor can be used instead of the older TIP30, transistor

All there other components are fairly common.
 
Notes.
Please note this is a completely custom board. While the board mounting holes correspond approximately to the positions in a Compupro box it is best to consider a completely customized S100 system with this board.
Also read here for a description of a similar 9 slot motherboard and here for an advanced S100 bus system by Jeffrey Wilson.
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.

To Order a Production S-100 Board
Realizing that a number of people might want to utilize a board like this together with a group of people on the  Google Groups S100Computers Forum, a "group purchases" is now OPEN. 
Please see here for more information, scrool dow to "New S100 Computer Boards" and fill in the form..  Please do not contact me directly.


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  12 slot board)   (V6.0  8/15/2021)
Most current KiCAD files for this motherboard       (for the  12 slot board)    (6.0  8/15/2021)
Most current Gurber files for this motherboard     (for the  12 slot board)    (6.0  8/15/2021)

  

This page was last modified on 08/19/2026