I've been working with 1970s devices and an unterminated system bus. I am adding termination to that but want to first understand a circuit that I have seen to make sure I understand what's going on. This is part of a schematic (the context isn't super important except that it's all 74xx series TTL). The DRFAL
and TR0*
signals coming in from the left are coming in over a ribbon cable from a connected disk drive mechanism, into a controller board (which is what this schematic is for):
Seeing the 220/330 ohm resistors in that configuration on those lines indicates a termination to me. I note that the signals continue beyond the resistor pair and into other logic.
Elsewhere, in this 1979 Kilobaud article "How Important Is Proper Termination" (regarding system busses), the following quotation and diagram are given:
"The standard TTL termination is a passive 2.6 V reference, composed of a 360 Ohm and 390 Ohm resistor in series across the power supply"
This shows a signal line "ending" at the resistor pair.
My questions:
In the first example, is it correct that the intention of those resistors is to provide termination of some kind on the signal coming in from the other device?
This seems like a dumb question, but is it the case that despite how they are drawn in this particular schematic (as if the signal continues "beyond" the termination), you could redraw this such that the resistors were at the "end" of the path, too-- thus that doesn't really matter, right?
The Kilobaud article says that a 360/390 pair is "standard". The drive controller circuit uses 220/330, for a 3V reference. Was that likely calculated from actual impedance of the line, or is this just another standard?
If my questions imply that I'm really misunderstanding anything or making bad assumptions as well I'm happy to know that.
Thanks!