However soon people wanted to increase the speed of the bus. Longer
multiple board slots soon appeared. It was not long until other electronic
engineers got involved and started producing "active terminated" signal
lines. The Godbout 19 slot S-100 bus board was one of the most popular.
This motherboard had pull up and down resistors on each of the data, address
and control lines to reduce signal noise. Later many commercial and homebrew S-100 systems had at
least 16-18 slots in a board cage to which multiple boards from multiple
manufactures were added. This became the standard home computer setup of the
late 1970's.
For people that have older S-100 motherboards that do not have any kind of
bus termination on the motherboard here is a simple board with termination
on the board itself.
The circuit (see
here) is quite simple, so the remainder of the board is
designed to be a prototype board where you can add your own circuits etc.
Once assembled you should adjust potentiometer R2 so the voltage on the
unloaded pins is 2.7VDC. That's right in the middle of "no man's land"
between HIGH and LOW for TTL chips.
The bus terminator should improve bus electrical
characteristics by reducing reflections and "bounce". It acts a bit
like a shock absorber or sound dampener.
Using 74LSxxx chips for bus buffers will help at lower
bus frequencies also due more gentle switching characteristics. The
trouble with many modern CMOS chips is they are too fast and lead to a lot
of overshoot on the leading edge of the square wave signal. The bus
terminator will help but not eliminate the problem.
Actually I was somewhat impressed when I looked at the signals of a simple
original Altair box with just a front panel, CPU, RAM and an I/O board. Here
is the clock signal (2MHz) before and after inserting the board. You
can see that after adjusting the resistor R2 the rising signal is a little
more "square".
I suspect at high speeds (8-12MHz) is an S-100 bus this board would be more
essential. Two MHz is not that high for a simple bus like this.
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, "group purchases" are made from time to time.
Contact and join the group if you would like to be involved in this
project. Please see here
for more information.
There is nothing particularly special on the board components except perhaps
the Op Amp.
The LM4250 is hard to find in 8 pin DIP packages these days. Digi-Key
has them (#LH4769, $1.00).
