NMEA Connections to ST2000+

[RiverRunner]

Hi!
I am using my ST2000+ for the second year now; the first year was stand-alone and it worked fine in that mode.
This year I added a Humminbird Helix 5 G2 GPS with NMEA 0183 capability. I have made the connections (Humminbird NMEA+ out to ST2000+ pin 6 and jumpered pin 5 to pin 2; Humminbird uses ground as the NMEA -; and enabled the GPS NMEA output) but the Tillerpilot does not follow a track or seem to in any way recognize the NMEA data.
Using a Logic Probe shows there is a regularly repeating data stream at pin 6 on the Tillerpilot socket with the GPS on.
In an effort to see what is going on, I connected the Tillerpilot and tried it with the GPS turned off. When I press -10 and +10 together (to enter tracking mode) I expected to get an error message saying 'no data'. Clearly, with the GPS off, there cannot be any. Page 51 in the manual shows a message like '- - -' should be displayed in either standby or auto; I do not get that. When I press the two buttons, I get a beep immediately and again in 1 second; the display never changes from showing the heading or set course (standby or auto). Likewise pressing +1 and -10 or -1 and +10 does nothing but beep. GPS on or off, for that matter.
In case it matters, I do have the operating sense reversed for my installation and have 'reset' the Tillerpilot according to instructions found elsewhere on this forum (set to 1000, cycle power, set to 2000, cycle power). All that did was force me to re-set the operating sense and force a compass re-calibrate.
Any suggestions on where to go from here? In stand-alone mode, the Tillerpilot continues to perform well.

RiverRunner
Bayfield 25