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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.
https://www.vintagecomputer.net/digital/TU56/TU56_PDP9-connector.png
https://www.vintagecomputer.net/digital/TU56/TU56_backplane-cards.png


Research/sources:

https://gunkies.org/wiki/TU56_DECtape_Transport

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.
https://homepage.cs.uiowa.edu/~dwjones/pdp8/models/

"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 uiowa.edu 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:
https://www.vintagecomputer.net/digital/TU56/TU56_PDP9-connector.png

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.

Considering
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:
http://www.bitsavers.org...aintenance%20Manual.pdf
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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