Hello Jeff,
I don't have Suuntto DC, so my answer will be pretty much a speculation, however I used to make custom electronics for IR communication with other DC so I think I can make a couple of educated guesses about your case.
First of all, I would like to point out, that any time FTDI is involved there're two connections at play. In general, you don't connect your DC directly to your PC, but you have some sort of custom cable or docker. This cable is not just wires with connector, you have USB-to-UART converter somewhere inside of your cable. At least most of the cables I've seen works that way.
There's USB connection between PC and FTDI chip and then there's another connection between FTDI chip and DC using serial protocol, that might be 3 wire connector or InfraRed interface.
The important part over here is that PC can only see first connection and when you tell: «Connection is established and Subsurface sees the computer.» that actually mean subsurface can communicate with FTDI chip, which is not DC, it's a chip inside your cable.
The reason I'm telling you all that is in most cases your PC or any software including subsurface, have no clue about what's going on beyond FTDI chip. Most likely, you can just connect cable to your PC without any DC attached and get the same error.
In your specific case, this is what I can see in the log. Configuration packages meant for FTDI got delivered then system expect some data to arrive from DC, but 3 seconds after 0 bytes have arrived. The key part is that it's total blackout. It's not a communication error or corrupted data, it's just no data at all. Like your DC is not even there.
[0.106991] INFO: Write: size=4, data=0F00000F
[0.115221] INFO: Read: size=4, data=0F00000F
[0.115225] INFO: RTS: value=1
[3.121940] INFO: Read: size=0, data=
[3.121944] ERROR: Failed to receive the answer. [in src/suunto_d9.c:253 (suunto_d9_device_packet)]
Now, here is what could cause this behavior:
- Something is wrong with your physical connection between DC and cable. Specifically the part/end which goes in contact with your DC, cause the side with USB is fine. Please, double/triple check it's connected properly and there's no debris, corrosion, e.t.c.
- Your DC is not expecting connection with PC. I do have DC which require me manually enter specific section of the menu, to get connected with PC and it doesn't response otherwise.
- Make sure DC's battery is not depleted.
- Your cable is damaged.
- Your issue is MacOS specific. This is not likely, judging by the log you've sent, but I cannot rule out this possibility. I gotta say I'm wrong person to make any suggestion on this matter, but most likely it has something to do with access permissions. Some sort of permission elevation might be requited to allow subsurface to communicate with hardware.
I really hope this will shed some light on the issue you're having.