I just solved the issue. Thanks for continuing to try and help. The problem was with the AIS unit configuration. While the manual states that the user cannot configure anything in the software this appears to not be so. After installing the ProAIS software and managing to connect to the device, I found out I can actually change baud rates on the device. Interestingly, the baud rate of the rs422 was set to 4800 and as soon as I set it to 38400, everything started working. The wire colours for this particular unit were black in the + and orange in the -, even though the manual seems to confuse the TXA with – and TXB with +. I don’t know why the baud rate was set up wrong on the unit because the user shouldn’t have to or even be able to configure anything.
All working like a charm now with your multiplexer. Thanks for the quick response and a good product.