S-100 bus termination

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: