Altair Repair

From: Dwight K. Elvey <>
Date: Mon Aug 2 12:03:08 2004

>From: "Marvin Johnston" <>
>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
>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 :).

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 )
Received on Mon Aug 02 2004 - 12:03:08 BST

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