PDP-11/70 debugging advice

Fritz Mueller fritzm at fritzm.org
Mon Feb 1 00:03:09 CST 2021

> On Jan 31, 2021, at 8:19 PM, Josh Dersch <derschjo at gmail.com> wrote:
> Well, what's interesting here is that on my system, switch S4 (MAINT STPR) steps the processor with switches S1 and S2 set to *any* configuration.

Hmm, would expect to see S2:1 S1:0 step by microinstruction, and S2:1 S1:1 step by clock phase.  The other two settings should free run the microcode.  So yeah, sounds like something fishy there...  The TIG card has more than a few analog components, and its not too unusual for these to get hung up on the adjacent card and have a leg pulled or sheared from the board.

> Ah, and page II-6-20 (p. 178) indicates that when DCLO is asserted, it asserts: "UBCE ROM INIT H - forces the ROM to ZAP.00 (200), and stops and clears the Timing Generator and the Cache timing."

Yup, that's one of the signals coming in to RAC E106.  Probing there should indicate which of possible sources for ZAP is actually occurring (UBCE ROM INIT H on pins 2 and 3 there).

DCLO is a classic...  Make sure to 'scope it, because it sometimes has troublesome spikes that don't show on a multimeter.  If you have H742s, there are some wet tantalums on the control board that sometimes leak and cause trouble with this.

I'm sure you are raring to go -- hope those fans show up for you tomorrow, and will be interested to hear what you find!


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