Better chat with Copilot led me to an important issue with the 1130 MRAM board - and it appears to be the root cause of my issues!

USING COPILOT TO REPEAT CONVERSATION WITH GOOGLE AI

I repeated my discussion from earlier, describing the observed signals and the sporadic failure of the SLT card AC trigger to recognize my sense pulse. Copilot quickly led me over to the drive of my pulse, which I described including the transistor type and expected sink current of 35-40ma. 

It then asked me details about how I was driving the transistor and pointed out that the 2K base resistor I was using would only provide about 1.35ma to the transistor which would NOT put it fully into saturation. That would mean that even with the minimum beta of 25 from the spec sheet, the transistor may not actually sink the 34ma I was expecting. Too little sink current would fail to pull the SLT input down low enough. 

That does match the signal trace I saw - I initially assumed the low level a bit below 1V was due to impedance in the path and circuit details inside the SLT card, but it very well could be a marginal logic level to trigger SLT. Copilot suggested switching to a 470 ohm resistor.

It then had a cogent discussion of how I was driving the base resistor - a 74LVC08A chip with 3.3V supply voltage. It confirmed that the chip has plenty of drive to deliver 5.7 to 7 ma to the base of the transistor. The chip is designed for up to 24ma per output. At the minimum beta, the transistor could sink up to 142ma of current but the SLT circuit will not deliver more than 1 ma or so. 

I even asked for an analysis of the current into the input pin of the SLT register card with the "AC trigger" - falling edge sensitive input. It gave a very nicely reasoned analysis, calculations and background. It also realized that the input was triggered by a falling edge, thus the capacitor is charged through the +3V rail on the SLT card. Gemini claimed it was charged by the -3V rail. 

Somehow when I looked at the spec sheet for the BVS52LT1G transistor I am using, I thought that my base drive current was sufficient. It doesn't directly give any number for what base current puts the transistor into saturation, which is the necessary condition for the transistor to fully drive the output. 

As you can see in the top line of the chart above, I should have a large signal (DC) gain or beta of 25 with 1ma draw. The saturation voltage for the base to emitter junction is 0.7 to 0.85 V and I was providing 3.3V through the 2K resistor. Naively I assumed the transistor was saturated and able to deliver the 25X gain from a base current of 1.23ma worst case - sinking over 30ma from the connection to the SLT card. 

DOING A TEST CHANGING THE BASE RESISTOR FOR BIT 2

Since the sporadic failure I keep seeing is on B register bit 2, I located that resistor on my PCB and swapped it out. I looked all over the shop for a 470 ohm 0805 size resistor - a very very common value. However, the nearest I could find was a 499 ohm resistor which is what I installed on the board in the bit 2 position.  I then fired up the machine and looped reading memory to see whether the errors disappeared. 

The machine ran for 22 minutes without an error on bit 2. It eventually failed when bit 0 was dropped, understandable since I hadn't changed any of the other 17 base resistors. I ran for a while with no probes attached at all - bit 2 was 'healed'. 

The pulses drive down further toward ground and are sharper to boot. I have a bit of ringing at the low point but the SLT logic is insensitive to such fast signals and was already triggered by the falling edge thus unaffected. 

The falling edge is FAST. The scope is set for 100 ns per division, showing the falling edge as essentially vertical, in the range of hundreds of picoseconds. It appears to fall to about 0.15V, far below the specification for a valid SLT logic low level. 

THE ROOT CAUSE APPEARS TO BE IDENTIFIED AND THE FIX IS EASY

Based on this test, it seems that I was driving a marginal signal into the SLT card due to inadequate base current on the fast transistor I have in the board. It was close but failed sporadically after a few millions of successful operations. Stepping up the base current 4X with the resistor swap gave me a very solid and reliable operation for bit 2. With the machine performing almost 278,000 reads per second, it completed more than 380 million successful reads before a different bit failed. 

I will replace all eighteen of the base resistors on my board. Once that is done, I will run the core memory tests and other CPU diagnostics for a couple of hours just to convince myself the memory replacement is solid. 

I DO AGREE WITH ONE OF MY READERS - COPILOT BETTER THAN GOOGLE GEMINI AI

This is the second time that I did a comparison of the conversations from Gemini (through the Chrome browser) and Copilot. At least for vintage technology subjects, It didn't take that much prompting to move the conversation to productive areas, unlike Gemini which just iterated apologizing and straying in new ways.