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.
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.
The relevant KiCAD files for this motherboard can be downloaded from the
link at the bottom of this page.
Here is a S100 bus label that can be used as a template on the edge
connectors using Avery® Clear Shipping Labels with Easy Peel® 7665 - 8½" x
11" labels. (Provided by Josh Bensadon).
A Custom 12 Slot S100 Motherboard with
Active Termination.
Please note there are
problems using this motherboard with CPU boards that utilize 74VLCxxx bus
buffers. All orders for the board are
currently canceled.
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:-
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.
There are a few minor modifications I made to the original 8 slot board.
For the Reset circuit:-
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.
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:-
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.
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, 5370, 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:-
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.
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.
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.
Please note there are
problems using this motherboard with CPU boards that utilize 74VLCxxx bus
buffers. All orders for the board are
currently canceled.
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.
