My Model M arrived last Thursday with a free adapter too but I dont use it anymore because it causes random chattering. But if plugged on PS2 port, no issue at all.
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A terminal requires more than an "adapter"
Oh! That is the reason why I am having problems when using the adapter.
I will give it a try later but this time I will plug it on USB2.0.
The OP showed a terminal Model M that requires a converter because it does not have native PS/2 protocol.
A standard M has PS/2 (or AT that can be "adapted" to PS/2) and does not need anything else. Some people have reported problems with cheap PS/2-to-USB adapters but they have always worked for me.
Model Ms do not have NKRO but I am not a rollover expert.
My experience is that USB 3.0 is often flaky with older gear.
At least for me, with, the popular blue cube does not like to be daisy chained to other adapters
When I convert AT to PS/2 and then PS/2 to USB via the blue cube, the keyboard does not function properly (it sometimes will work for like 5 seconds)
I also tried USB to PS/2 then back to PS/2 and it still didn't work (there was some serious lag between keystrokes and half of them didn't register)
However, straight PS/2 to USB works fine.
BTW, couldn't you just buy a cable from another Model M with a detachable cable and then use it in place of the SDL to RJ-45 cable on a terminal M? Or does it have to do with the firmware on the board.