Probably Entrex has special cables made that did not connect pin A on one end to pin A on the other, but made the necessary connections so that for example the B/F pair (outgoing data) was routed to the D/H pair (incoming data). This would allow several terminals to be "daisy chained" together, provided that each terminal passed data along that was not addressed to it. Note that the trapezoid terminals have a set of switches to set the terminal identity -- presumably each terminal would be set to a unique identity.
But we are getting ahead of ourselves. Let's look at the 14 signals, which the schematic identifies using letter from the alphabet (for pins on the connector). The letters G, I, and O are not used. For the trapezoid, it is most useful to talk about pairs of pins, since all the signals are differential pairs. I also give the signal names that I am using on the schematic
D/H -- CG_data_in - input data (goes directly to serial in of 8273 shift register) J/P -- CG_clock - input, looks like the clock N/R -- CG_inD - input, looks like the clock, but I have doubts A/C -- CG_outA - output, uncertain B/F -- CG_data_out - output data (comes directly from serial out on 8274 shift register)So that accounts for 10 of the 14 pins, here are the rest:
E -- to duplex switch K -- unused L -- unused M -- unusedThe E pin that goes to the duplex switch is a mystery so far, and I am leaving it for another day to sort out.
A/C -- input data, differential pair N/R -- output data, differential pair P -- output (single) transmit request H -- input (single) transmit enable L -- output (single) "MBZ" monitor busy B, D, E, F, J, K -- ground M, R -- unusedNote that the MBZ signal on pin "L" is hidden in the middle of sheet 2 and I missed it until AJ told me about it.
Here only the input and output data are differential pairs. Along with these we have a handshake set up (using plain old single ended signals) so the host can tell the terminal when it is permitted to send. The terminal can tell the host that it is busy using the MBZ signal.
Tom's Computer Info / firstname.lastname@example.org