UNIBUS extension card/cable sets

John Wallace johnwallace4 at yahoo.co.uk
Fri Jun 26 14:58:42 CDT 2015

[huge snippage for brevity, apologies for rubbish formatting]

I'm not 100% sure I'm right in what follows (it's been a long time) but improvements are welcome. It builds on much of what has already been said.

Termination has been covered by various contributors - termination reduces (but may not completely eliminate) reflections.

Reflections in a single segment setup (two boxes, one cable) are relatively simple to cope with in a setup from the Qbus era where things aren't particularly fast.

In a three box (two cable) setup my recollection is that the configuration rules require the cables to be of significantly different lengths, and the reason for this is to ensure that the two sets of reflections are timed signifcantly differently and canot make Bad Things happen by arriving at the same time as each other.

Consider the middle box (of 3) is driving a bus transition. Signals will propagate from the middle box to each of the ends. When the moving rope er sorry voltage transition reaches the end of the cable, it will be reflected to some extent. If the cable segments are both the same length(ish), the reflections will come back to the middle box at round about the same time, superimpose on each other, and potentially cause confusion. (Does that sound plausible?)

If the cable segments are of significantly different lengths then the reflections will arrive back at the middle at significantly different times and the reflections will be more manageable - less risk of Bad Things happening when they superpose.

Or something along those lines.

Anyway, hopefully the "different cable lengths so the reflection timings are different" will ring a few bells even if it's not actually right.

Have a lot of fun
John Wallace

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