19 Jul
2013
19 Jul
'13
3:23 p.m.
On 2013 Jul 19, at 8:03 AM, Dave wrote:
Now, if for some reason the Init/ signal is never nbeing asserted, the
CPU microcode program counter will not be cleared. I think, though, that
no matter where the microcode starts, it will end up in the fetch-execute
loop -- but the CPU registers will not have been cleared (thsi is done
by the power-on self test microcode), so it's unlikely to run BAISC
correctly. Maybem though ,the keydown routine, particularly if you
pressed STOP. does intialise enough to get a display
To the OP : After pressing STOP, does there appear to be any progrma in
memory?
I have a 9830A that I haven't powered up in a while. Previously,
it would take a few seconds to get to the lazy-T prompt, but now on
powerup, the screen remains blank. I got Tony Duell's phenomenal
schematics, and was all set to start debugging, starting at the
clock, but I checked the user manual first, and following the
suggestion in the setup section, I hit "STOP", which brought me to
the prompt, and from there, it appears to be functioning
normally. However, the machine powers up this way every time--
never to a prompt.
Does anyone have an idea why the machine is behaving this way? I'd
love to have it power up to a prompt.
As Dave suggests, one place to start would be to check the power-on-
reset circuits. The main POR circuit for the 9830 is a 74121
monostable on the power supply board. The signal it generates is
available on pin 3 of the little 12-pin service connector situated
between boards 12 and 13 on the backplane. The signal is supposed to
go, or stay, low for around 100mS around power-up, after which it
goes high, releasing the processor to run.
There are actually two POR circuits in the 9830 though. The second
one is in the tape drive circuitry, on board 62. It may be that if
the tape drive POR is sluggish, the CPU is hanging on the tape drive
during the initialisation process.
I think this is unlikely. The trivial rason is that in genral capacitors
dry up, so that the POR signa lwil lbe too fast, not too slow. The main
reaso nis hat I can't see anyhting hat ath signal (ClrMark/ in my
schematis) could not clear that STP/ would then clear.
It is worth checkign the KeyDn/ interrupt signal after power-on. If it's
asserted and stays asserted you need to find out why.
I have been reverse engineering the 9830 recently (a couple things
left to clean up in the schematic), and Rob and I are currently
I am sure htere are plentyy of other machiens htat need documentation...
working on a project regarding the 9830/9865 tapes.
Between all that
One day I must get roudn to gettign my Digideck to read nad write such
tapes. It's muc hthe same mechansim and the encoding scheme is so similar
(even down to the marker bit i nthe middle of each byte). and I ahve the
full manual for said drive.
-tony