Altair Repair

From: Marvin Johnston <marvin_at_rain.org>
Date: Mon Aug 2 12:37:50 2004

"Dwight K. Elvey" wrote:

> 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.

Since this does appear to be related to the databus and getting the
correct information to the CPU, I'll start by checking to make sure the
JMP and NOP instructions are getting out on the bus. What is interesting
is that the switch settings seem to have almost no effect on anything
until I turn them all on. But the JMP and NOP buffers could well cause
some problems. Wish I could find my Jade Board quickly :). Right now,
I'm using a Bob Mullens S-100 extender card with the High, Low, Pulse
LED indicators ... may have to actually fire up the scope if I don't
find out the problem fairly soon.

> 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 )

This is one of the original style Altairs where the wiring between the 4
slot motherboards and the front panel are all done with discrete wires.
Makes me wonder if the 2 MHz clock speed is too fast :). I've checked
out all the data and control lines from the front panel to the bus and
everything seems to be correct. I did notice that the Phase 2 clock
doesn't register on the Bob Mullens card, but that may be because the
pulse length is too short for the card to read (123 w/ 10 pf cap and
about 6.? K resistor.)

Thanks!
Received on Mon Aug 02 2004 - 12:37:50 BST