Friday, November 14, 2014

Assembling SAC interface box

Hectic day today, including the followup to my root canal, but I did get some time to finish up the socket wiring. Tomorrow is the last day I will be home to work on the system for a couple of weeks, as I will fly out early Sunday morning. I will do some design work and VHDL coding while away, but there won't be all the opportunities I have while here.


I continued to make ground signal wires, add the female pins, insert into the socket and twist the ground and associated signal wires together. Although the CHI interface terminated all the ground pins to a common ground bus bar right at the socket, the IBM products that use these cables maintain the twisted pair right to the point of attachment at a backplane or card, leaving each ground of a twisted pair isolated until the closest point to the driver or receiver gate.

I chose to implement my interface according to the IBM approach, terminating the ground wire of each signal pair on the interface board next to its receiver or driver circuit. It had the side benefit of cleaning up the wiring between the socket and the interface cards, with each twisted pair remaining more distinct than the forest of individual wires that would exist if I bonded the grounds at the socket.

All the signal lines have their ground wires installed and twisted around them, but I haven't figure out what to do about the remaining six pins - do I install these as ground wires hooked to a common bus? I need to look at what is done inside the 1131 and how CHI handled it, to make a decision.

I made brackets to mount the four interface cards vertically one above the next, just in front of the socket inside the enclosure, with airflow in line with the cooling fan that sits to the side of the socket. I did a test run soldering the five signal pairs that hook to the bottom board, the one with only five driver circuits.  I will begin soldering the remaining signal pairs onto the assigned boards tomorrow, when the sun is up, since I prefer to do the soldering outdoors on a folding table in front of the garage.

Bottom board (4) soldered in, board 3 mounted waiting for its 24 twisted pairs to be soldered

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