1130MRAM smoking gun found

SCOPE ON 3.3V AND GROUND, TRIGGERED BY THE ANOMALY

I set the scope on the yellow trace and triggered on the first fall of the +Storage Read signal, which will occur during the glitch that surrounds the spurious retriggering in the read timer chain. I wanted to look extremely closely at the 3.3V line at the time of the anomaly, watching very closely for any variation that suggests bouncing on the ground or 3.3V lines that feed all the chips.

I set up the green trace to a wire plugged into a ground pin in gate B compartment A1, in AC mode with 200mV per division sensitivity. The blue trace was hooked to the +3.3V side of a huge capacitor I added to bolster the 3.3V supply stability, also on 200mV per division in AC mode. The purple lead was connected to the output of the second read timer to watch for retriggering and thus duplicate pulses. 

Because I suspected that I might have some bounce in the supply, I had added a 46,000 uF electrolytic in parallel with the 470uF filter capacitor I previously added. Even with all that buffer capacitance I was seeing the retriggering occurring. 

When I looked at the 3.3V output at the buffer capacitor, I saw a horrifying bounce of 800mV up and down. 

Watching the 3.3V supply bounce along with the second read timer output pulse, the correlation is obvious. 

Watching the ground pin on the IBM 1130 to compare its level to the ground at the huge filter capacitor on my board, I saw about a 700mV +/- bounce as well. 

This is a classic analog issue, lying beneath the digital abstractions of logic gates. There is likely some kind of resonance or ringing at play here, looking to be around a 1-2 ns cycle duration. My suspicions are going to be aimed initially at the power traces to all the chips, but I will also look into the chip bypass capacitance values I chose and its interplay with the board and trace capacitance, resistance and inductance. 

I suppose I will also route the traces much more carefully to round every bend and avoid any spots that could produce reflections, since this board designed for 2.5MHz signal rates is having more difficulties that I had anticipated. 

It seems inescapable that I will have to buy another round of PCBs to fix all of this, incurring the cost and delay.