Fixed my wrap plug so that it works properly with the 3174

 IBM WIRING DIAGRAMS DISCUSS WRAP PLUG AS NOTES

Each of the three cable types (EIA, X.21 and V.35) have their own wiring diagram in the maintenance manual. It shows which signals are wrapped back or joined together when the cable is switched to Test mode. 

There are notes at the bottom which state that the EIA wrap plug is wired the same way as the cable with Test turned on, but with two differences. It stated that Request for Test is also looped to Ring Indicator, and that the cable ID pins 14 and 16 are wired together but neither to ground. 

The V.35 diagram notes state that the wrap plug is exactly like the effect of all three switches on the cable set to Test, but with the cable ID pins 14 and 16 wired together but not to ground. I had always wondered how the microcode differentiated between a V.35 wrap plug and an EIA wrap plug.

MY ORIGINAL TRY WAS TO MAKE AN EIA WRAP PLUG

I wired my breakout plug exactly like the EIA diagram for Test switched on, including the extra loopback and the change to the ID pins 14 and 16. When I tried to run a wrap test with the diagnostics, it failed with an error that suggested it didn't see the plug as a wrap device. I tried connecting 14 and 16 to signal ground but the result was the same. 

I had then moved to wiring the breakout board exactly like the EIA cable in Test mode, without the extra circuit and having the ID pins set as for the cable. That had worked just fine, but it nagged at me that I didn't have a working wrap plug wiring on the breakout plug. Now that I have a working cable and don't need my breakout for most operations, I decided to look more closely at the wrap plug situation.

DISCOVERED THAT THE WRAP PLUG IS A COMBINED EIA/V.35 COMPONENT

I finally noticed the IBM part numbers on the diagram for the wrap plug were the same for both EIA and V.35 which told me that it might be a superset of both wiring diagrams, taking into account the notes from both. I looked to see if there would be conflicting wiring in the breakout if my theory was correct. X.21 cables, on the other hand, have conflicting pin usages which is why IBM has a unique part number for the X.21 wrap plug!

Interestingly, there were no conflicts. It turns out that the wrapped signal are almost totally on different pins on the DB25 connector, thus I added all the loopbacks and joined wires from the V.35 diagram in Test mode to the wires I had in place for the EIA in Test mode. I put in the Req for Test to Ring Ind circuit and set up the ID pins 14 to 16 with no ground. That was the merged requirements from both diagrams and their respective wrap plug notes. 

Joint EIA and V.35 Wrap Plug

TESTING THE NEW WRAP PLUG PROVES MY THEORY, ADAPTER PASSES TESTS

I installed the new wrap plug and brought up the diagnostics from the Utility diskette image. When I ran the wrap test (test 01 for hardware group 11), it ran to successful completion! The code displayed at the end of the test was 2011 which for the hardware tests is interpreted as successful test end 20xx and hardware group 11 for the unit under test. Thus my wrap plug is now correct.