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Negibus or Posibus?

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  by Bill Degnan - 01/17/2023 15:29
I am working to understand this TU56 as it came from an unknown environment. The apparent peripheral connecting cable (a W850 C (~1966)) that came with the TU56 reads "PDP9" on one end and "TC02 E&F" on the other. I wondered if this cable is actually associated with the TU56 or just sitting in the same box before the TU56 came to me.


On this page, one can see photos of the backplane. Note how a TU56 could have had a M531 or a G742 installed to serve as a bus converter. THe M531 is for buses with negative logic controllers and the G742 is for when one has a positive logic controller.

Jumping a few steps ahead in thought, if the TU56 was attached to a PDP-9, would you expect the TU56 to have a G742 or M531? This would help prove whether the TU56 was indeed used with a PDP-9. Note that newer PDP8 and PDP11's were positive bus.

I found a good general description of how the negative / positive relates to the PDP-8 and how peripherals fit into the picture.

"The "negibus" or negative logic I/O bus used -3 and 0 volt logic levels in 92 ohm coaxial cable" - a M531 would be used if attaching to a computer with a negative bus. A G742 indicates the computer using the TU56 had some form of positive logic bus.
The positive I/O bus, or posibus, was a 100 ohm bus clamped between 0 and 3 volts with TTL drivers and receivers.

Does this mean, no this TU56 would not be compatible with a PDP9?

Not so fast! - The W850 C (~1966) cable that connects the TU56 to the external peripheral possibly appears to convert the posibus to a negibus as described in the above link. So, I am thinking the presence of a G742 does not alone indicate the computer using the TU56 must have a positive logic bus. It might simply be that someone tried to attach a "new at the time" TU56 to an "old at the time" PDP-9, around 1976 and they needed to make due with whatever cables were available. It would have been easier to use a straight-through cable with a M531, but these were not available? Just a guess. Note the 18 connectors are present:

PDP-9 and earlier PDP8's were negative logic computers. Thus a TU56 with a G742 would have needed the W850 C to make it compatible with a negative logic computer, the PDP-9 to operate correctly. I think.

1) We have a 18-wired jumper bus adapter (W850 C) on the other end of the TU56 positive bus that I believe would convert the signal to negative
2) THe "PDP9" printed on the adapter.
3) The other side of the cable reads "TC02 E&F" (I believe).

I know PDP-9's attached to TU55's, but when PDP-9's manuals were printed the TU56 was not yet invented. I did find that TU56's could be used with TC02 peripheral backplanes (such as an expansion box).

TU56 and TC02:
See page 22
(1) For a TU56M or TU56MH Transport: controller-to-transport information cable = 70-06412, transport-totransport (other TU56 or TU56H in system) information cable = 74-5152. See Note 2 for TU56M or TU56MH connected to a TD8/e Controller. (2) When a TU56M or TU56MH is connected to a TDa TD8/e Controller, only one 70-08447 cable is used for both commands and information. This cable has both a single-height and a double-height connector module on the termination end. Insert the single-height connector into slots A061 A07 and the double-height connector into slots AB 1 01 AB I I.



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