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An S-100 Bus Terminator & Prototype Board.  
It is easy to look back now at the early Altair S-100 computer and see how it could have been improved, we should remember however, that when Ed Roberts in 1974 started there was severe price constraints of what the system could cost.  It was this low cost that got the home computer concept going amongst hobbyists and launched the industry.

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. 
  
  Terminator Board
  
 
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 squarewave 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".  
 
  Before & After Termination Board
   
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.  See if bare boards are available and/or see if you and others may be interested in doing another board run.
 
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). 

The 270 Ohms 5 resistor pac is Digi-Key #NC-15281
The 270 Ohms 9 resistor pac is Mouser # 652-4610X-101-271LF. (Can be found at Dig-Key as well).

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 SMB BOARD SCHEMATIC 
(FINAL, 5/18/2013)
MOST CURRENT SMB BOARD LAYOUT 
(FINAL, 5/18/2013)
Most current KiCAD files for this board    (S100 Bus Terminator-001.zip    11/5/2014)


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This page was last modified on 05/14/2016