DSC NMEA between a97 and ICOM M504 VHF

[DenisStyp]

I upgraded my boat electronics and replaced two C90w MFDs with an a97 and a98. I rewired my ICOM M504 to the a97. NMEA Out from the a97 goes to the VHF NMEA In (+ to +; - to -). NMEA Out from the VHF goes to the a97 NMEA in for port 1 (White/Green pair, + to +, - to -).

I have enabled DSC alerts on the MFD menu (Lighthouse II release 17). The Baud Rate is set to 4800 for Port 1 on the MFD. The GPS data is sending to the VHF fine; LAT/LONG displays on the VHF. However, DSC Position Reports received by the VHF display properly on the VHF, but the signal doesn't pass through to the MFD to pop the DSC alert window.

To troubleshoot, I wired a loopback on the a97 MFD wiring harness (Input to Output). The a97 diagnostic buffer showed TX and RX. Good. I reconnected the wires on the a97 to my shielded 2-pair that runs to the radio box. I removed the wires from the VHF, and wired a loopback in the shielded 2-pair at the radio box end. I checked the MFD and saw RX/TX data in the buffer. So, my MFD and wiring to the radio box appear good.

I rewired the shielded 2 pair to the radio box using a terminal block. With the MFD and radio on, I placed a 2-6V LED from radio shack across the terminal block +/- screws for the MFD Out to Radio IN. As expected the LED pulsed for the data being sent. I then placed the LED across the screws for the radio out/MFD in pair. I initiated a Position Request from a friend. The radio received it. The LED never blinked, data never went to the MFD.

At this time, I suspect the OUT NMEA circuit on the ICOM to be bad.
Do you have other ideas, or do you agree?