Revised design and producing a new board for 1130 MRAM project

IN ORDER TO CONTROL THE CURRENT SINKING, I MOVED TO DISCRETE TRANSISTORS

Since the bounce is dependent on the number of the sense bits that have a 1 value and thus cause an output pulse, the issue is in the open collector NAND gates, the PCB itself or something the IBM 1130 side is driving into my board. I decided to use a NPN transistor to sink the current from the 1130 to produce the sense pulses - which are negative going pulses from the 3V SLT pullup voltage down to ground. 

The pulses are in the range of 80 nanoseconds in duration, so I had to use a transistor that was fast enough for this, would recover back to 3V quickly and could sink the 8ma I estimate that is pulled to set the 1130 flipflop with a 1 value. I selected the BSV52 from OnSemi. It has turn on and turn off times of 12 and 18 nanoseconds, able to sink more than three times my target current and readily available. 

With the transistor I had to flip the output gate from a quad NAND open drain (74lcx38) type to a quad AND gate (74LVC08A). These use the same TSSOP-14 footprint and pin assignments. 

I redesigned the PCB to introduce the 18 transistors and 18 base resistors between the sense output pins and the quad AND gate. Thus when the read timer goes off with a positive pulse of 80-100 nanoseconds, any RAM bit that is one will cause that AND gate to output an 80-100ns positive pulse. The BSV52 transistor will invert this with its open collector to pull the 1130's 3V sense output line down to ground for 80ns. This will be offset by about 12 nanoseconds but that is well within the wide range of timing that is available to set bits into the 1130 Storage Buffer Register. 

I was very diligent to make sure that the ground plane was uninterrupted except for small cutouts around vias, thus offering a very strong path for the current to sink into the PCB from the transistors and out the heavy stranded wire to the 1130 power system. 

SWITCHING TO NEW FAB WITH PCBWAY

I received an offer from PCBWAY.com to sponsor the PCBs I develop in exchange for my sharing the experience and board quality on this blog. Later when I assemble the new PCB to begin testing, I will share a few paragraphs covering PCBWAY. I took the opportunity to buy an SMD stencil, which should make my soldering much easier.

An SMD stencil is a thin piece of metal that has holes cut in it for every pad. Solder paste is spread across the stencil and is thus only deposited on the pads themselves on the board. My manual method of applying the paste left excess which turned to teeny solder balls that spattered around, while also being inconsistent enough that I couldn't guarantee a good connection of every pin. 

HOPED TO VERIFY THE SOLUTION WORKED BEFORE PRODUCING THE PCB

I wanted to breadboard some transistors to see if that would resolve the issues with spurious retriggering of the timer pulses. That was going to involve my bodging up another PCB blank, then using spare SLT ribbon cables to jumper this special board between my PCB and the 1130 system. I figured out a way to use the blank board to hold pins to form SLT sockets. 

I would be able to form two sockets, to handle eight sense output pins. One socket would connect to cable T4 from the 1130 and the other socket would plug into a jumper cable that is in turn plugged into my current PCB position T4. I would tack wires on pads connected to the pins and then run them to discrete transistors on a perf board. Signal quality wouldn't be great but I could validate the plan to work properly without the retriggering. 

Because the existing PCB has a NAND gate output, I would have to invert the signal with one transistor and then drive the open collector connection with a second. Thus the perf board would have 16 transistors and a bunch of resistors, all tenuously wired to the pads of the special PCB that acts as a pair of SLT connectors. 

However, a similar discrete NPN transistor with these low delay characteristics is obsolete, thus would cost several dollars per transistor and be tossed at the end of the test. The surface mount BSV52 transistors are way too tiny to try to tack on wires to the leads as it would be a very unreliable mess. 

I therefore had to gamble that this will work, ordering the BVS52 transistors and new PCB up front, then assemble it and only then see if the fix works.