PDP-8/I Negative-bus termination

[Josh Dersch]

On Wed, May 5, 2021 at 12:27 AM Vincent Slyngstad via cctalk <
cctalk at classiccmp.org> wrote:

> 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.
>

Thanks!  Looking at the engineering drawings and other docs for the RF08 (
http://bitsavers.org/pdf/dec/pdp8/disc/DEC-08-HIEA-DA_RF08_Jun70.pdf):

Section 2.2.5 of the manual states "Figure 2-5 shows proper cable
connections between the RF08 Disk Control and the central processor and
proper termination techniques at the time of installation."  In figure 2-5
(p. 25) there's the cabling diagram and no explicit termination
instructions called out for slots D01-D06; however, looking closer at the
IOP signals, the lines coming out of the block terminate in a circle, as do
the T1, PCL, and B RUN signals.  Is this circle DEC shorthand for
"terminate this?"

Additionally, on p. 88, there are a series of asterisk-denoted 100 ohm
resistors (connected to the IOP1/2/4 signals) which I assume are meant to
be on the backplane.  On other schematic pages there are other 100 ohm
resistors (with asterisks) for T1, PCL, etc.  These don't appear to be
present on mine; there's a note on p. 88:

"* 100 OHM TERMINATING RESISTORS ARE USED ONLY ON THE PDP-8 NOT ON THE
PDP-8I"

which would likely explain it.  Based on this, I /think/ I'm OK termination
wise with my system. Wonder why there's a difference between the two
systems...

- Josh



>         Vince
>