Re: 2445 EPROMs

Chuck Harris

Hi Chris,

My humble apologies, the CPU uses a 5MHz square wave clock. The
processor is a 68A02.

Early microprocessors usually took several cycles per instruction.
5, I think was the minimum for the 6800 family... leading to a 200ns
requirement on access time.

It has been a long, long, time since I last had to design with, or
debug a 6800 series microprocessor. I have never found one bad in
all of the 2465's I have calibrated... actually, I have never found
one bad in anything I have worked on.

200ns is what tektronix used as the access time for the RAM, and EPROM.

I would put the probably that the EPROM is good at 99.999%... unless
you have had a power supply problem that has elevated the 5V to above
7V... in which case, most everything on the CPU card will be bad.

Look instead at the jumpers on the board. There are test jumpers that
will prevent boot up.

Also, look at the reset circuitry. A single bad part, and the CPU
will never get out of power on reset. Pay particular attention to the
0.1uf and 2.2M resistors that provide the timing.

Assuming that there is CPU activity, there should be an LED lit on the
front panel indicating the failure cause.

-Chuck Harris

christopherbath@... wrote:

Hi Chuck,

Looking at the schematics the processor seems to be driven by a 5MHz clock (the clock being generated by a 10MHz crystal). According to the datasheet for the 6802 the maximum clock frequency is 4MHz so I am not sure what is going on here? Possibly the processor is a MC68A02 or MC68B02. I guess you would need to look into the bus timing is more detail to work out the access time requirements for the ROM but agree that 250ns or less would be more than enough.

The 2445 won’t power up properly. It seems to me as if the processor hangs on start-up. Given that the scope seems to do slightly different things on start-up I am suspicious about the EPROMs that would have been programmed about 30 years or more ago. The power supply rails all seem to be ok so I think the EPROMs are the next place to be looking. Perhaps I should also check the clock and reset for the microprocessor.



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