behaviour of classic PDP-8 frontpanel

[Klemens Krause]

Is there anybody out with a working classic PDP-8?

For a long time we had the problem, that starting a program on our 8 by 
pressing <LA> <START> keys, this program crashed. Examining the memory 
contents showed, that typically one or two words short behind the starting 
adress after such a crash had inadvertently content 0000.
For a long time I suspected a subtle memory problem. But now after 
carfully having revised the memory timing I hopefully found the real 
reason for this nasty misbehaviour:
if the machine is running and I press the <START> key, it stops and
there is also the chance, that one word in memory is nullified.
A look in the maintenance manual shows, that pressing <START> clears the 
memory data register and others asynchronously. Naturally if this occours 
in the moment, when a memory read is in progress, data from core has been 
transfered to memory data register, which clears the word just have been 
red, and before the memory timing chain was able to write this word back 
to memory, then this word is cleared out.

Scoping the <START> signal coming from the front panel shows heavy 
bouncing of this key: <START>ing a program produces in many cases more 
than one <START> pulse. The first starting the program, the following in 
some cases nullifying a memory location near the start address. The 
problem showed only, if the second start pulse came just in the moment
between reading and rewriting the word in core. In the other cases
program just went on.

My workaround for this problem in the moment is a 22uF electrolytic capacitor
between the <START> signal line and ground to make a "brut force" debounce 
of this switch.

Now my question to other classic-8 users: What does your machine do, if a 
program is running and you press one of the front keys, which shouldn't be 
pressed while a program is runnig: <START>, <DEP>, <EXAM>?
Logically it makes no sense to press these switches if the machine is 
running.
On later machines (8/L, 8/E) these keys have no effect when <RUN> is on.

Maybe that at that time the keys were new and didn't bounce and the 
designer simply said: you must not press these key while the machine is 
runnig.
Or there is a kind of feedback to these keys which inhibits them if the 
machine is runnig and which is defunct in our machine. But I didn't find 
such a key-inhibit line in the maintenance manual.
Sadly in the manual I also didn't find a complete diagram of the 
frontpanel. Neither in that one on bitsavers nor in our original
printed version.

May one of the classic-8 users here in this group make the experiment: 
Start a program
an press <START> again?

Thanks in advance
   Klemens