NEW APPROACH IN LATEST PCB DESIGN

The prior boards used an open collector NAND gate to generate the 80-100 ns pulse that represents a 1 bit at a given bit position in the addressed word of memory. When several positions had a 1 value, spurious retriggering would occur which produced a string of pulses rather than the single pulse that should be emitted during a read cycle.

The new approach places a discrete transistor as the open collector driver of the pulse, after being fed by an AND gate (since the transistor inverts, the NAND logic function must become an AND). This isolates the current sinking that occurs during the pulse and ensures a pure ground path for that current which comes from the IBM 1130 logic cards for the Storage Buffer Register.

I selected a transistor with very small delays in order to crisply produce the pulse - BSV52 - but there is no discrete version of this, only surface mount parts. That makes it quite difficult to test out the use of the transistor without having a PCB onto which it will be soldered.

METHOD TO ATTEMPT A TEST BEFORE THE NEW PCB ARRIVES

Since PCB fabrication takes some time, both for the manufacture and then the shipping, I thought of ways that I could test the theory in the otherwise idle days ahead. I realized that I could intercept the connections and interpose transistors between my existing PCB and the 1130, by repurposing a blank of the old PCB type and two IBM SLT ribbon cables.

The IBM 1130 connects to my 1130 MRAM PCB with three SLT ribbon cables (T1, T3 and T4). My PCB has pins on it that form an SLT male socket into which the connector end will plug. I realized that I could form SLT male sockets on a spare PCB using more pins, then grab the signal on the pads where they would have connected to integrated circuits if the board were fully assembled.

I will check this out using only cable T4. This delivers the pulses for sense bits 10, 11, 12, 13, 14, 15 and the two parity bits P1 and P2. I will plug cable T4 into the spare PCB position for T1 and plug a spare SLT ribbon cables between the T4 slot of the spare board and the T4 slot of my 1130 MRAM board. Cables T1 and T3 run from the 1130 directly to my PCB.

The spare PCB will have the sense signals from my board arrive on position T1. I will grab the signals with wires tacked onto the IC pads connected to the pins of T1 that I want to intercept. Those signals go to a breadboard with a pair of transistors for each bit position. The first will invert the signal to convert the NAND output of my PCB to an AND output. The second will be the open collector driver of a signal connected to position T4 of the spare board again using IC pads that connect to the relevant pins of the connector.

BUILDING THE SPARE PCB TO INTERCEPT SIGNALS

I tacked wires on the pads to capture the eight signals from connector T1 and to output signals onto connector T4. I then began installing the gold pins that form the SLT male socket into which the ribbon cables connect.

However, I ran out of pins and have to wait for additional pins that are on order from Digikey. Meanwhile I can work on the breadboard with transistors and do some testing of the capabilities of the discrete transistors I have on hand.

BUILDING BREADBOARD WITH PAIRS OF NPN TRANSISTORS

I used a very simple circuit for each bit position I am intercepting. A first NPN transistor has a base resistor of 2K and a 10K pullup resistor on the collector to 3.3V. The input from T1 on the spare PCB needs a pullup to 3.3V as well because my original PCB is open collector, expecting the 1130 side to pull the line to 3V. The second transistor is coupled to the collector of the first by another 2K resistor. The collector is what is hooked up to the T4 side of the spare PCB.

I had a number of discrete NPN transistors on hand. I tested with both 2N5551 and MPS2222A transistors in pairs. I used a square wave generator as the input and observed the output on my oscilloscope with an added pullup to 3.3V on that probe.

LIMITATIONS OF THE DISCRETE TRANSISTORS I HAD ON HAND

The 2N5551 mangled the square wave at pretty low frequencies and stopped working at 750 KHz. My pulses are equivalent to more than 10 MHz. This certainly won’t work with the 1130 system.

When I tested the MPS2222A transistors, things got even worse. The transistor only worked up to about 140 KHz and the wave was much more mangled.

The BSV52 should do the job, with turn on and turn off times of 10 to 12 nanoseconds while the discrete transistor types I used had much longer times. The nearest discrete part I could find that was similarly low in delay is 2N2369 or 2N4449 but these were not on hand. I just bought some from Mouser and will retest in a few days when they arrive.