Thanks for the tips Allen. I am not issuing any commands to the Nexus server. By default, it starts publishing NMEA data at 4800 baud. All you have to do is read it. (If you want it to publish in its high-speed, proprietary binary format, called FDX, you have to issue a command to the server by writing a special string to the port).
The commands I posted are issued on my Linux computer. The first one sets the baud rate of the port, and then I am reading the port using the normal Linux utility 'cat'. It's when I run the 'cat' command that the BTW display on my instrument immediately goes to "- - -".
I may try writing my own program and do non-blocking reads as you suggest. I have never checked the CPU usage, but I don't believe it is pegged at 100%.
If that does not work, I'll investigate getting rid of the 'write' pin on the cable.
Will also check out StartLine.