Testing the 1130 MRAM core memory replacement on the IBM 1130 system - part 2

WATCHING SIGNALS FROM THE 1130 TO THE BOARD

My first check was to watch the +StorageRead, +StorageWrite and +StorageUse signals as well as -BBit0 where I could verify that the LOAD mode of the rotary mode switch was delivering the intended bit value. 

I was missing one of the key signals, +StorageWrite,  from where I expected it to arrive. However I did see the -BBit0 signal do the right thing based on the console entry switch settings during a LOAD. 

The next test watched both a -Bbitx and a -Sensebitx line to verify their pullup to +3V when not activated. These were pulled up to 3V on the bit lines I checked.

PLUGGED IN MY BOARD, WHICH DID NOT WORK CORRECTLY

One of the symptoms was that the 1130 recorded a word with bit 10 on any time I did a DISPLAY, which flagged a parity error since my board was outputting the parity check bits based on whatever was coming out of the memory chip. These would always be correct which meant that the 1130 was detecting differently than I was outputting. 

I still had the missing +StorageWrite signal issue to contend with. I decided to test continuity of all the signals from the source gates in the CPU out to my board - all 16 -BBitx, all 16 -Sensebitx, the two parity check -Sense bits, and the three input control signals +StorageRead, +StorageWrite and +StorageUse.

CONTINUITY CHECKS OF THE SIGNALS ARRIVING ON CABLES T1, T3 and T4

The Automated Logic Diagrams (ALD) are the source I used to document the cable pins associated with each signal to the memory gate B-C1. I found discrepancies from what I expected! 

The +StorageWrite signal was only detected on cable T3 at pin J1 A11 which I had listed as a duplicate of the signal on H1 D11. When I tested the continuity between those two pins while everything was plugged into the memory compartment, they were tied together. 

However, the connection between the two could have been implemented in one of three places - the compartment holding the source gate could send its output to both pins, the cable itself would have tied the two wires together, or the memory compartment could have tied the pins together on the SLT board where the cable plugs in. 

With the cable disconnected from the destination SLT board in the memory compartment, the pins were not tied together, thus the connection was done on the memory compartment SLT board we are replacing. I chose the wrong pin of the pair that are connected in the memory compartment. 

In an ALD, the gate producing a signal has a list at the bottom of the page of every pin where the signal is connected off this ALD page. For the +StorageWrite signal, the gate producing it is AY on page MC101. The list of off page destinations was:

This only shows connections from compartment 01B-B1 where gate AY is situated to other pins on 01B-B1, not to the memory compartment. However, you need to understand that the locations in the top row of any compartment are the T1, T2, T3 and T4 cables that connect to other compartments. Looking at the cable drawing below we can see that the last two pins on the list are part of cable T3 that runs from 01B-B1 to 01B-C1 (our memory compartment. 

Thus the signal from gate AY in MC101 goes to pin H1 E11 of the memory compartment through cable T3. There it is also tied to pin J1 A11 but only on the SLT board in compartment 01B-C1 which we are replacing. I chose the wrong pin to route on my PCB. 

In addition to the control signal, which was a major error that blocked correct operation of my board for any memory access, there were five more signals where a pair of pins were tied together in the memory compartment yet I picked the wrong pin of each pair for my PCB. These are:

• -BBit0 which I assumed was B1 A09 but instead was only routed to A1 E09 on the cable
• -BBit10 which I picked from L1 A09 but was wired only to K1 E09 on the cable
• -Sense Bit 3 which I picked as B1 D11 but was wired to A1 E11 on the cable
• -Sense Bit 7 which I picked as C1 D11 but was wired to C1 A11 on the cable
• -Sense Bit 13 which I picked as L1 D11 but was wired to K1 E11 on the cable

I can tack a wire on the back of my PCB to hook the pairs of pins together as they are tied on the memory compartment SLT board, which will resolve the issues. In my defense, I saw the signal duplicated on the cable and picked the pin that fit the pattern of the others, but that was the wrong choice electrically.

You can see above how the -BBit0 and -Sense Amp Bit 3 pins I chose are in line with the others and fall into a nice pattern, while the first two instances at the top seem out of place. 

When I get back to the workshop, I will rework the back of my PCB to tie together the six pairs of pins where I chose wrongly - every single duplicate I found by the way. Then I will test again.