Thursday, June 2, 2016

Continuing design for use of EMM 32K word memory module


I spent the morning at the dentist and afterwards it was just too hot to work in the shop. Hoping to resume work tomorrow morning when it is cooler.


There is some ambiguity in how the interface is to be wired, at least as far as terminators for the twisted pairs coming from me to the memory module. The address lines are 15 or 16 single ended inputs and if they are terminated also, the math gets funny

I would have 52 candidates for termination but only 43 terminators. I had to investigate further to understand this. Fortunately, I figured out why I don't need all 52 signals terminated.

Looking at the schematics, I found three signals (COOP, TIOP and MSOP) that wouldn't need termination, as they pulled up to high in the PCB and are only statically set to configure how the board should work. Additionally, the MP (memory protect) and GR (general reset) signals are designed for startup and powerdown, not during operation, so they dont' need termination.

Selecting the board uses three select signals (MS1, MS2 and MS3) but the board provides three inverters so that the incoming signal can be inverted if that makes sense for the particular board. If the inverter is used, the terminator is on the input to the inverter (XA#) and no transmission line is needed to connect the inverter output to the MS# input. This eliminates three more signals.

As a result of the above, 8 signals were and I can ignore the high address bit when using this in its 32K word x 18 bit mode instead of 64K x 9. Still short a couple of terminators, probably there are 2-3 signals that are static in a full system. Fortunately for me, I am only working with an 18 bit word length and the basic design is for a 20 bit word. Thus, I can skip terminators for data input bits 19 and 20.

I planned out the pin numbers for all the twisted pair signals, plus the terminator pins to tie to each input pin. The remaining task was to tie all the grounds of the twisted pair to the 7 or 8 ground pins on each connector.

I will check the board against the pin assignments, as a sanity check to be sure something isn't off with the documentation.

Time to begin wiring up the connectors. First will be the terminator connections to the input pins. Next will be all the twisted pairs side A to the input pins, and hooking the side B of each pair to some common ground lines. Finally, I will wire up the power supplies to the connectors.

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