KY11-LB (pdp11/04/34) "Documentation error" & block diagram

[intabits]

After cleaning up and sorting out the PDP11/04/34 power supplies, I'm starting to fire up my two machines with boards inserted - first off, just the M7859 console interface and the M9302 terminator board.

I knew of the documentation error in the manual EK-KY11B-MM-001, regarding the orientation of the cable connecting the console and the M7859, but didn't refer to the error discussion, figuring I'd just make sure that the red wire went to pin 1 at both ends. And if it I'd got it wrong, it would be obvious because all four status LEDs would be on, and it doesn't cause any damage anyway.

So I powered it up and the console didn't work, but only one status LED was on, so I didn't immediately suspect a reversed cable. Turns out that's what it was, plus a bad M7859, M9302, and the 11/04 console, which were all easily detected because I have two of each of them.

But before I discovered the reversed cable, I figured I'm in for some troubleshooting, and began drawing a block diagram with connector pinouts, as an aid to future debugging, based on the engineering drawings (MP00015_KY11-LB).

In doing this I discovered that the drawings indicate that J1 pin 1 on the console connects to J1 pin 20 on the M7859 (although no pin numbers for J1 are used in the drawings for M7859 - they use the DEC alphabet A..X instead of 1..20), but pin A does go to physical pin 1 on the boxed header.

So the error is really in the design, being that the two PCBs use incompatible pin assignments, requiring that the IDC socket at one end of the cable be inserted backwards, so that pin 1 at one end connects to pin 20 at the other. This means at least one of the cable IDC sockets must be physically non-polarized so that it can be inserted backwards. The documentation would be correct if there were no design error, but of course it is wrong because it should reflect the workaround needed to overcome the design error.

Anyway, for what it's worth, here is my block diagram of the KY11-LB console, in case anyone else finds it of value:-

High Res PNG is here:-
https://imgur.com/y3t35yd

 

[intabits]

(Unable to edit the original post)

I've made some corrections, changes, and additions to the diagram:-

HiRes:-
https://imgur.com/aGfOSIl

 

[wa2flq]

I remember coming across this back when I had a PDP 11/34. The cables running from the controller to the KY11-LB laid pretty flat in the chassis. I had DEC gear with more than a few data cables no physical polarization as well.

Before marking it up as an "error", I wonder if it was intentional as part of cable management?

 

[intabits]

Before marking it up as an "error", I wonder if it was intentional as part of cable management?

For this particular cable, I can't see this as anything but an error.

The placement of the J1 connectors on these two PCBs requires that the cable have a right angle fold at a minimum - fine. If the cable enters the console area horizontally over the top of the frame, that's it - a neatly done cable routing.
But these cables seem to be routed so that they enter the console area from the left, resulting in more and tighter folds. Plus the cable is more than twice as long as it needs to be, with all the excess length being stuffed into the cavity next to the fan, making it all a bit of a mess to begin with.

To then ensure that pin1=pin1, one additional twist would be required. And whether entering from the top or the left, there is ample room to accomplish this. With all the other folds and stuffing, the twist would be easily incorporated and barely noticeable.

Generally, I think that it's perfectly reasonable to expect that cable pin 1 should go to pin 1 whenever possible. And when there is a very good reason why that is not possible, physical polarization should be used, rather than relying on the user having some special knowledge that this particular connector must be fitted backwards. I don't think that avoiding a single twist where there is plenty of room for it, in an already messy cable route is sufficient reason to make such a non-standard connection.

Rather, my guess is that they were not trying to avoid a twist in the cable, but by the time it was realized that there should be one, it was too late to fix the PCB layout, and the pragmatic decision was made to "just plug one end in backwards". But I still think they should have polarized the cable to ensure this was done.



I don't know if anyone sees a use for my diagram, but hopefully they do, here is a further minor update (plus an alternate view of the console section):-

HiRes:-
https://imgur.com/ba3xuMt
https://imgur.com/Z08GZ60