PDP11 Hack Plus board, part 2

So my PCBs arrived (I opted for black, just because) and I built one up run tests. Now the 6402 and the DCJ11 are in sockets as they’re hard to find.

The two buttons are Reset and Halt, as I couldn’t fit all the LEDs and their driver chips on the board I opted for breakout pins for things I thought I might need to monitor.

First problem was a 6402 that was faulty and as a result killed the 1.8432MHz module, somewhat annoying, but in this day and age of chip shortages and fakes sadly not unexpected, so swapping out another 6402 and a new clock module and the clock module runs, YAY!

However I’m unable to get anything from the board, specifically I can’t get the ODT up, which according to the website is a combination of reset and halt “at power-up one should press the RESET switch, press the HALT switch, release RESET, then release HALT”. However nothing happens.

So I popped on the logic analyser, fearing that the DCJ11 may be dead and when I power it up, I can see some activity;

ok, so the ALE line is working (it latches the address from the multiplexed data/address pins (DA0->DA15) and latches the address, awesome. I can see it’s trying to to do something with memory (the MEM line is high).

So, I try the reset/Halt thing, and I get exactly the same.. so the processor is restarting, but seemingly not going to the ODT, it’s always running this exact sequence at the start after a reset.

Pressing Halt on it’s own should bring up the ODT, but nope, nothing happens i.e. the data and address lines don’t change at all from the value they settle at after the initial boot… interesting…

Feeling a bit dejected I’ve left it for a while, what I think I’ll try next is to check the IO line (instead of the MEM line) and also the enable lines for the 6402 UART chip, I’m curious to see if it’s trying to do anything. It’s possible the DCJ11 is fried, or at least partially fried, but without a known good one to test against I can’t be sure.

If anyone out there has any more experience with this, I’d love to talk to you 🙂

Anyway, more to follow next week.

electronics and synth nerd.