Found these inside a non-working 71B

[Paul Berger (Canada)]

(03-14-2018 06:19 PM)MikeSD Wrote:  I'm still puzzled about why the RAM didn't work on my unmodified 71B. I did not have HPIL attached when testing.

But there is something different about the original unit it works in, which might explain why in it and not an unmodified one.

I've attached a pic below of the back of the PCB on the one that works. The U9 (I think) bottom chip, pin 9, has a jumper to the Math ROM (top chip, pin 8), which one might expect, if expanding to that top chip spot.

But that U9-9 also goes to pin 10, which picks up the trace to HPIL, when the top chip slot isn't needed. Not sure why it needs to go to pin 10 at all. If they wanted to also go to the HPIL, they could have done it without going to pin 10.

The only reason I can see for the line to be routed this way is for convenience of laying out the PCB, in any case I would not get too wound up in why it is this way.

(03-14-2018 06:19 PM)MikeSD Wrote:  That said, the trace is broken where it connects to the HPIL trace. So, since U9-9 doesn't go directly to HPIL any longer and the trace is cut, how does HPIL get back in the loop. AND, might this extra wiring and cutting, be the cause for the RAM not working in the unmodified unit.

Ok so now Dout (pin 9) from the Math module is going out to Din (Pin 8) of the first 32K module the first is daisy chained to the second RAM that is hung off the wires. Dout from the second 32K has to be connected somewhere to complete the chain.

If you have the FORTH module connected it would go to Din on the FORTH module and the Dout from the FORTH module would go to the pad on the board where Dout from the module that is in the Math position would normally be connected, That pad should be connected to Din on the connector for the HP-IL.

If you do not have the FORTH module connected then it would be the Dout from the second 32K that would get get connected to that point on the board.

The trace that is cut effectively bypasses the module position where the Math module is installed as far as the daisy chain is concerned, by making that cut you isolate the Dout of U9 from the trace going to the HPIL connector, so that it is only connect to Din for the position where the Math module is, and only Dout for the position above U9 is connected to Din on the HPIL, so the to complete the chain, Dout from the last module before HPIL need to be connected to the Dout pad for the position above U9.

(03-14-2018 06:19 PM)MikeSD Wrote:  And now that I think about it, the rest of the chips is how it gets back to HPIL. I think pin 10 is just connected but not used, or possible for some other reason. The chain goes from Math, to RAM, to FORTH, THEN to the HPIL.

Dang! Still don't have an explanation for why RAM didn't work in unmodified unit.

Yes pins 10,11,and 12 are not connected to anything on memory modules, so the board designer could safely route signals through those connections.

No idea why it would not work in your test 71B if everything was connected correctly, Dout from U9 should have been routed directly to the HPIL so connecting it Din of the first 32K module should have worked.

Paul.