help - 11/34 console problem

[Tony Duell]

> 
> Hi all,
> 
> I finally got juice on the main power line connection in my museum.
> I am connecting my machines bit by bit, after 2 years of inactivity.
> I have found the following problem with my 11/34C.
> (I will keep the description short, and tell only the end result)
> 
> The console of the 11/34C (11/34A plus cache option) shows weird
> (=wrong) behaviour. It was working fine!
> The display shows "000000", and the RUN LED is off.
> If I press the 'LSR' button, the 'SR DISP' LED goes on.  However,
> if I press any of the numerical buttons, '1-6', a digit in the
> display only flashes very brief. I cannot say what the digit was.
> It looks like the M7859 has developed a problem.
> To make things more weird the following happens if I start with
> pressing the 'CLR' button and then the numerical buttons.
> 1) press CLR (display stays at "000000")
> 2) press e.g. 6 (some of the rightmost digit segments briefly flash)
> 3) press 6 again (1st and 2nd of the right display flashes)
> 4) press 6 again (1st, 2nd and 3rd of the right display flashes)

This suggests it is correctly counting the digits you enter (or shifting 
them into a register), but is not displaying them correctly. And it is 
only updating the bottom 8 bits.

As you probably know, the console is controlled by an 8008 
microprocessor. Of course it's an 8 bit device. Maybe some 'carry' is not 
working correctly.

Alas DEC didn't publish a source listing (or even a binary dump) of the 
console firmware in the printset. This is going to make troubleshooting a 
little harder, since you probably don't know what it should be doing. It 
is clearly running a program, and that program is reading the keyboard 
and scanning the display. So it is somewhat correct.


> 
> The first leftmost three displays remain "000" and do not show any
> sign of changes.
> Pressing 6 after the first 3 times, repeats the flashing of some
> segments in the three rightmost displays.

What does it do if you press 4 repeatedly? Does it flash the bottom 3 or 
bottom 2 digits? If it's only updating the bottom 8 bits, you see, then 
only 2 digits will flash from 0.

> 
> So, I have the impression it has the problem to do with the M7958.
> AFAIK, the entry of the digits, for example the numeric entry '165020'
> does *not* need the UNIBUS, but only sets up an internal register.
> The 'LAD' button, and 'DIS AD' will start a UNIBUS cycle (to access a
> memory location).  Correct?

I think so. 

> 
> BTW, the voltage on the CPU backplane (DD11-PK) is 5.08 V. and the
> +15 and -15 read +/-15.x V. (Can't remember the exact value).
> 
> I have an other 11/34 with an M7958, but never powered up that box.
> I took that M7859 and installed it in the 11/34C. The display stays
> dark. According to the M7859 doc, that basically means that the board
> is dead.  So much for board swapping.  Sorry, could not resist to try
> an easy success.  I don't have other spare M7859 ...

You may find the totally dead board easier to debug. At least you can 
check for a clock at the 8008, and for activity on the internal bus to 
the firmware ROM, etc.

-tony