Altair Repair - test-drb@ccmp.vtda.org

2 Aug 2004

...
 From: "Marvin Johnston"
&lt;marvin(a)rain.org&gt;

I fired up an Altair and found it didn't behave as it should (tested
with MITs and Polymorphic CPU cards.) Doing an "Examine" does not load
the switch settings into the program counter. Doing an "Examine Next"
seems to increment the program counter by 3. The preceeding was done
with and without a memory card. I tried the "Deposit" but had no way to
see what I was doing since the switch settings apparently weren't being
loaded. Finally, the "Deposit Next" increments the LEDs. One other
strangeness, when I connect the Data Display cable to the MITS card, the
"Stop" switch ceases to function although the "Start" seems to work
fine.

I do have the schematics and will start to dig into it later this
morning. BUT, I was hoping to short circuit some of the troubleshooting
time if someone on the list has experienced something like this :).
Thanks!

Hi Marvin
 It sounds like the front panel isn't jamming the correct
instructions onto the data bus ( or something on the data
lines of the CPU is loading one of the data lines ).
The front panel works by either jamming a JMP ( 0C3h ) instruction
onto the CPU's bus or a NOP ( 00h ). This allows for both changing
address and incrementing the address. The front panel can then
override the read and write operations to the physical memory
during the increment.
 The most common problem I've seen that causes similar effects
is that there are several 7406 ( or similar OC buffers ) connected
...
 from the front panel to the data lines of the CPU. One
or more of these devices has failed ( I suspect this is mostly do to
some overlap timing and stressing of the parts but that is
the way they are designed ). One other thing that I've seen
is that the data lines going to the CPU from the front panel
were wired upside down. This is that cable between the two.
( soemone put the cable together backwards )
Dwight