I think this is on-topic due to my hypothesis about the Namco board.
JVS IO have 2 sense lines - one is input from next IO in the chain, and second is output to host or previous IO
in the chain. and they are not connected. JVS Host have sense input only.
I understand there are 2 directions for Sense for handling multiple nodes, and I understand they're not connected.
I'm referring, for this topic, exclusively to the side of the I/O referred to as an output connected to the motherboard, in the instance where there is a single I/O. In that case the side of the I/O that senses Sense from the previous I/O is irrelevant.
Even though it's referred to as output on the I/O, in my experience it is NOT outputting anything or doesn't have to, at least on my custom I/O, which has proven to work on NAOMI, Chihiro and Lindbergh. It is only connecting voltage coming in from Sense, dropping the voltage through diodes and sinking the line to ground when needed.
Here's the diagram from the manual I'm using for how I've implemented it:
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zaFVbU+QMXioo6/w9pI4s6gUK7woq6QOCqijr/D2kjizqBQjv/I6z9/bvMizp/4EJdUecvz/w/A38+YAeBdqPRSP7EbduWnsGB+RlGFDhDTrbwFt/KD//NCiH8ZzoF3kINAu2IzYO0IxRA2hEKIO0IBZB2hAJIO0IBpB2hANKOUABpRyiAtCMUQNoRCiDtCAWQdoQC/g+mEiqIRT7CLgAAAABJRU5ErkJggg==
B connector is where the Sense voltage comes in from the main board. The I/O brings it to ground when the transistor is activated and drops voltage across 4 diodes, basically affecting the Sense line as far as the main board is sensing it.
So in this case, the I/O is NOT responsible for sensing voltage. It just drives the transistor to ground Sense during initialization.
This is why the adapter Darksoft has put together should not require Sense. The I/O should not care that Sense is present on the B port.
What I'm suggesting is that on the Namco I/O in question, perhaps they integrated some kind of sensor at the B connector to make sure Sense is present from the motherboard before allowing acknowledgements to commands.
We'll know more once
@xodaraP puts an adapter together for the extractor. If no replies, then maybe apply 5v to Sense and see if it replies then.
*Edit: If applying 5v to Sense, do so through a 10k ohm resistor. Do not wire 5v directly from a power supply to the Sense line, the I/O will short it to ground during initialization.