Mark

429 data bus output going to three locations.

Do you have 429 reader?

I would see if one of the user units has a bad 429 receiver (which I've seen before) which is locking up the entire data bus.
Disconnect one to see if the other two come up. If not, hook the first one back up and try another. Try it again on the last unit.

Is either of those data bus wires shorted to shield or ground??

Do any displays show annunciation or range marks that would indicate the data "is" reaching the display, but not the actual weather? If that was the case then you would need to go back upstream to the antenna input.

It looks like you've got a jumper that tells the unit to send the output to EFIS, is that present?

What is PN342 IAPS???