PDP-8/I Negative-bus termination

Vincent Slyngstad vincent.slyngstad at gmail.com
Wed May 5 02:27:18 CDT 2021

On 5/5/2021 12:03 AM, Josh Dersch via cctalk wrote:
> Until this point I've never had any peripherals for my negibus systems
> (apart from teletypes), and it occurs to me that I have no idea if the bus
> needs to be terminated (and if so, with what).  There are 6 slots in the
> RF08 backplane (D01-D06) for daisy-chaining to the next device, which is
> where I assume they'd go; the RF08 manual does not make it clear what this
> looks like or if it's actually required, and I've gone through the
> available PDP-8/I docs and I'm still at a loss.
> Can anyone with negibus experience point me in the right direction?

I don't have any actual Negibus gear except a TC01 (which I've yet to be 
able to cable to anything), so I'm likely not the best expert.

Regardless, my understanding is that it is terminated in basically the 
same way as the positive bus, which is to say you stick a G717 in the 
BAC connector that has the IOP and TS signals on it (at the far end of 
the bus, of course).

This has the effect of terminating the signals that need to be cleanest 
with 100 ohm resistors to ground.

Not sure where I read that though, so I'm not able to easily double 
check my memory.


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