Replacing some chips on 1130MRAM board based on testing

TOGGLING STORAGE READ AND OBSERVING THE SENSE OUTPUTS

I have the board set up with a switch that will drive a rising edge on the +Storage Read line, causing my board to emit a sense pulse for any data or parity bit whose value is a 1. The output has a pullup resistor to +3.3V and a sense pulse consists of a short pulse to ground for less than 100 nanoseconds, occurring roughly 800 ns after the rising edge of the triggering signal. 

SUSPICIOUS RESULTS

When the memory chip was not yet installed, most sense bits appeared to be a 1 and produced pulses, but not bits 2, 3, 4, 13 and 14. The two parity bits produced pulses when odd parity existed on the eight bits they covered - those were correct based on which bits were generating pulses. 

SEVERAL CHIPS COULD BE THE CAUSE

I removed the buffer chips to see if they were holding down the five erroneous bit positions. This is a tri-state buffer that will drive the data line on the memory chip when we are doing a write, but floats to let the memory chip produce an output during all other times. With the chip removed, all the data bits appeared to be a 0, thus it was the tristate outputs of the buffer chip that were sensed as a 1. 

The data outputs feed into a NAND gate which produces the sense pulse when the data bit is 1 and the timer chain drives the output pulse at the correct time in a read. One more chip is connected to the data lines, an XOR chip that is used to calculate the odd parity and produce the parity bit value. 

Thus, the cause for bits 2, 3, 4, 13 and 14 to act differently could have been the NAND gate, the XOR gate, the buffer chip or the memory chip. The memory chip driving an output during a read, the other three chips having a bad input that pulls down the data line. 

REMOVING VARIOUS CHIPS TO NARROW DOWN THE CAUSE

I desoldered various chips to help identify which chips needed replacement. With the memory chip uninstalled but the others there, I observed the problems. When I yanked the buffer chip from one location, the problem bits from that side seemed to go away. When I soldered down the memory chip but had the two buffer chips removed, the results were still mixed. 

There were some differences from the prior condition, but not all bits acting the same either 1 or 0. That could be the contents of data in the chip that are driving those results, but it could also be the XOR chip inputs causing the problem or the NAND outputs.

I had changed the NAND chip that produced bits 12, 13, 14 and 15, but still had the same results in the earlier tests before the memory chip was installed. However, I could have an issue with the chips that output 2, 3, and 4 - two different chips as one handles 0-3 and the other covers 4-7. 

ORDERING REPLACEMENT CHIPS

I have spare NAND chips on hand, but no spares for the buffer nor the XOR chips. I placed orders with Digikey so that I can swap in known good chips in these positions and continue my testing until I am satisfied that the sense outputs are working properly on the test bench. Only then will I move over to the 1130 and test with this cabled into the system.