I have been running some tests and learning a lot about this little machine and its ways.
It has two layers of self-test. The first is a mandatory Power On Self Test (POST). The second layer is an optional interactive test made available by setting switch 4 on the main DIP switch block and activated by the toggle on the back panel (replaced in part 1)
In my second post I couldn’t get to the optional test no matter how I set switch 4 inside and the toggle switch on the back. Instead I was getting an octal number and being dropped into the ODT.
I naively thought that this was some kind of RAM value. It wasn’t. This was in fact an error code from the POST. The value I was given, 170732 means there is a RAM fault on the main board. I have spent some time re-seating all of the socketed chips and I think that’s what fixed this.
So the reason I couldn’t get to the optional test suite was that the POST was failing.
Past the POST.
Now that the basic POST is passing, I can get beyond that to the attempts to boot I showed in the last post.
At this point it doesn’t make any obvious attempt to access the disk drives. I would expect a little graunching to find track 0 or something similar but there is nothing.
I can now try setting the DIP switch and toggle to enable the test mode. With this done and a reboot I get nothing on the terminal at all. Flick the toggle for normal mode and it’s back to the boot prompt. Set the toggle to test and nothing.
The PDT-11 has a number of status LEDS on both the main board and the disk controller. Setting for test mode again, opening up the lid and separating the boards shows us a number of clues.
A quick switch later and the error code on the LEDs has changed to 0010 which means the MSB is now faulty.
These chips are 4bit x 256 bits and some new ones are on order. 🙂