PDP-8/I Negative-bus termination

Michael Thompson michael.99.thompson at gmail.com
Wed May 5 15:49:44 CDT 2021

> Date: Wed, 5 May 2021 00:03:05 -0700
> From: Josh Dersch <derschjo at gmail.com>
> Subject: PDP-8/I Negative-bus termination
> Hey all --
> 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?
> Thanks,
> Josh

The DEC Field Service Technical Manual has some notes on bus termination.
It says:

Termination is required on I/O cables longer than 20 ft., and may be
desirable on shorter cables. For negative bus, use 220 Ohm shunt resistors
to ground on IOP 1, IOP 2, IOP 4, BTS 1, BTS 3 and Initialize. No special
termination module exists for the negative bus.

Michael Thompson

More information about the cctech mailing list