Runt Pulse on data line

[don.r]

I am getting a runt pulse on a data line. Chips involved are 6803MPU, 6521 PIA and 1024x4 static CMOS ram (6514). Aside from swapping to known good devices, how do I narrow it down?

EDIT it isn't the RAM because it occurs on all data lines (RAM only occupies first 4).

[srb1954]

I am getting a runt pulse on a data line. Chips involved are 6803MPU, 6521 PIA and 1024x4 static CMOS ram (6514). Aside from swapping to known good devices, how do I narrow it down?

EDIT it isn't the RAM because it occurs on all data lines (RAM only occupies first 4).

Presumably you have an EPROM as well since the 6803 doesn't have internal program memory.

Check the outputs of your address decoder to see which device is activated and whether it is a read or a write cycle. This should help determine which device is driving the bus and whether it is a bus contention between the processor and another device during a write cycle or just a weak drive from one of the bus devices during a read cycle.

Not all runt pulses on a data bus indicate a real problem; it might just be that nothing is driving the data bus at the time and the lines drift to some intermediate voltage. Put some pull-up resistors, say 10k, on the data bus to see if the runt pulse still appears, indicating that something is still driving the bus.

[don.r]

Yes, there is an EEPROM but it checks out fine. Everything operates as normal save for one of the outputs of the PIA. I may be chasing a ghost with the runt pulse and might have to do some part-swapping. First, I will break out the MDO and check the lines and see what is happening during the runt. It is a highly repeatable event so whatever is causing it is not intermittent.