The last and final update:
I feel stupid for taking this long to discover the issue but it required an oscilloscope to finally eliminate all errors.
As mentioned before, the AIS unit was a version 1 NMEA talker. This was easy to determine because it has a TX line and a ground line as the output. My Raymarine plotter uses Nmea version 2/3/4 nomenclature with a TX+, TX-, RX+ and RX- lines. Unfortunately, behind the curtain is a version 1 talker/listener. When hooked up to the RS422 to USB converter in the expected manner, the signal was just barely decoded by the converter. Actually some converters didn't decode it at all and some converted it mostly ... but with an error here and there. Errors might show up almost immediately or not at all over a 24 hr test run. Even temperature seemed to influence the error rate.
Once I realized I was dealing with the old NMEA protocol and hooked it up correctly to the converter, it seemed very solid over a 24 hr run with no errors.
Due to the intermittent nature of the errors, I can't be 100% certain that everything is working properly but I am 99.99% sure that all is ok!
I feel stupid for taking this long to discover the issue but it required an oscilloscope to finally eliminate all errors.
As mentioned before, the AIS unit was a version 1 NMEA talker. This was easy to determine because it has a TX line and a ground line as the output. My Raymarine plotter uses Nmea version 2/3/4 nomenclature with a TX+, TX-, RX+ and RX- lines. Unfortunately, behind the curtain is a version 1 talker/listener. When hooked up to the RS422 to USB converter in the expected manner, the signal was just barely decoded by the converter. Actually some converters didn't decode it at all and some converted it mostly ... but with an error here and there. Errors might show up almost immediately or not at all over a 24 hr test run. Even temperature seemed to influence the error rate.
Once I realized I was dealing with the old NMEA protocol and hooked it up correctly to the converter, it seemed very solid over a 24 hr run with no errors.
Due to the intermittent nature of the errors, I can't be 100% certain that everything is working properly but I am 99.99% sure that all is ok!