S-100 bus termination

From: Joe <rigdonj_at_intellistar.net>
Date: Sat Jul 25 02:05:58 1998

Doug,

   Since I got my Altair, I've been boning up on the s-100 stuff. I got an
active terminator board with my Altair so that's one thing that I paid
particular attention too. Here's what I found out.

   The first S-100 computers did not come with any termination but they
soon found that the long bus lines were causing ringing and false
triggering in the circuits. Ringing is caused when a signal reaches the end
of an unterminated (open) line such as a data buss.* The signal reverses
polarity and travels back in the direction that it came from just like an
echo. When it reaches another end, it changes polarity and direction again.
Now you have two positive pulses and one negative pulse where there is only
supposed to be one pulse! Consider the number of times that each line
branched off to a card socket and you can understand that now there where a
dozen or more echos on every line and they were all at different times.
It's no wonder that the systems where so difficult to make work with all
the noise on the buss.

   The first solution was to use passive termination to try and eliminate
the noise but it had limited success. This method used resistor divider
networks that connected to the 0 and 5 volt power with the junctions
connected to the signal lines. They had to keep the resistances high to
minimize power supply loading and to prevent clamping the line so well that
no signal could get through. This method wasn't entirely effective due to
the combination of high clamping impedence (resistance) and unterminated
branchs (sockets).

   The next solution was to use active termination. This method uses what
amounts to small individual power supplies on each line to hold the line
voltage at a level between 0 and the maximum siganl level (5 volts). This
method offered a lower clamping impedence AND less power supply loading and
was better able to prevent the ringing.

  The active terminator baords were a very popular accessory and a lot of
people where making and selling them. In fact, I have an article from one
of the first Byte magazines about how to make your own active terminator
board. I believe that the later S-100 systems like the N* Horizon came with
termination built into the MB. The ringing problem in the early Altairs
was made worse by use of the long wires that ran from the MB to the front
panel. Better designs eliminated a lot the sources of the ringing. FWIW
this is EXACTLY the reason that you still have to terminate SCSI busses and
disk drive cabling. There is still an ongoing debate in the SCSI community
about the nessecity/benefits of active vs passive termination.

* OK OK! Just for you purists, ringing is caused ANYTIME a signal makes a
transistion from one impedence to another. The bigger the change, the
bigger the reflected signal. That's why when ****power**** or signal
distortion counts, you have to try and match all the impedences in the
signal path. That means that your signal source such as an antenna, the
signal destination such as your TV set and the interconnecting cableing all
have to have the same impedence or you have to use a matching transformer
to match the impedences. Go look at the back of your televison, you'll
usually see a matching transformer and 300 Ohm and 75 Ohm inputs. The two
inputs are there so that you can use the one that matches the impedence of
the lead that brings the signal into your home. Most TV antennas have a
300 Ohm Impedence. The round coaxial cable has an impedence of 75 Ohms. If
you are using it then there should also be a small matching transformer on
your antenna.

   Joe


At 07:26 PM 7/24/98 -0500, you wrote:
>I was digging through some S-100 cards and came upon a Godbout active
>terminator. Do all S-100 buses require external termination, or did more
>have it built into the mobo? Specifically, do I need to stick this card
>into my IMSAI?
>
>-- Doug
>
>
Received on Sat Jul 25 1998 - 02:05:58 BST