Thursday, November 23, 2017

Worked out wiring for 2621A terminal


2261A terminal debugging

Relatively useless set of pins since it is on the connector on the logic board, not on back of terminal
In the wiring of my cable between the serial board and the terminal, the output Data Terminal Ready is looped back as inputs on Clear to Send and Data Set Ready. The other two inputs are Read Data, of course, and Ring Indicator.

The outputs are the aforementioned Data Terminal Ready, as well Send Data, Ready to Send and an ignored signal to select the alternate speed on dual speed modems. What is missing is proof that these eight signals are wired to the Centronics 50 connector pins I expect, located on the rear of the terminal.

One possible cable connection to the 2622 terminal, similar to 2621
The 2622 is very similar to the 2621A that I have as my primary terminal and identical to the alternate terminal. I try to use the cable diagrams like the one above, but these don't match exactly the signals used by the 12966A.
Cable defined between 2621 terminal and 12966A serial card
The above diagram is the one in the manual for the 12966A serial card that claims to be the wiring to the 2621A terminal. I do indeed have those wires connected as listed to the Centronics pins. For example, pin 12 on Centronics is the Brown wire which carries Receive Data from the terminal to the board.

Note that a jumper is defined at the bottom between Centronics pins 36 and 46. It turns out that 36 carries +5V and it forces the Secondary Channel signal to on. The other wires line up reasonably well except that the middle diagram above shows pins 16 thru 23 of the Centronics connector are shorted together.

In the theory of operations discussion for the terminal, it mentions that a signal DM indicates that a connection was detected, causing an asterisk to appear in the middle of the bottom line of the display. This is the loopback from the Data Terminal Ready output, through the cable. 

I went out to scope the connections between logic board J6 and the Centronics 50 connector, as well as studying the state of pins 16 to 23. I discovered that J6 on the logic board is a 2x17 connector strip seemingly wired in a pseudo-1-to-1 manner to the Centronics 50,

That is, only 34 of the 50 pins on the Centronics are wired. Pins 1 to 17 on J6 are wired to pins 34 through 50 respectively of the Centronics, while pins 18 to 34 of J6 go to the other side of the Centronics, pins 9 to 25.

The result is that the Centronics pin numbers don't make sense compared to the wiring chart from the 12966A diagram. What I know for sure is that the J6 connectors are correct based on the processor schematics. Using the logical function of the lines and the purpose of the wires in the cable to the 12966A board, I figured out how the Centronics connector must be wired.

How I figured out the proper wiring for the terminal

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