If you want to read speed over ground (SOG) course over ground (COG) and few other things on your Raymarine ST60 Speed display, you will need to first connect the data output of a GPS receiver to the NMEA input of your Raymmarine ST4000 Autopilot. If you can get that working the Autopilot will then transfer this data over the Sea Talk interface to the ST60 display. Note that the ST60 does not have a NMEA input.
Also if you ever expect the Coast Guard to find you when you issue a DSC distress signal you will need the NMEA interface to your DSC radio to receive the current GPS coordinates.
I was having difficulty getting the ST4000 to listen to the NMEA input. The RAY54 UHF DSC radio was OK, and my RADAR display an SL70 was getting the GPS sentences. The only solution I could get from the Westmarine Raymarine expert was to spend $3500 on a new Chart plotter and a completely new Autopilot system.
After sifting though a lot of misinformation regarding NMEA that I found on various sailnet blog sites, I solved the problem. Turns out that prior to Raymarine version 2 NMEA was RS-232 (+12 to -12V) and after that it was changed to EIA-422 (0-5V). Knowing what version you vintage equipment is does require trial and error. All that stuff about NMEA interfaces where + is talkers and - is listeners is a bunch of hogwash. + is positive and - is ground. Its that simple.
As part of Raymarine's unending quest to ensure that every few years your perfectly good nav equipment becomes obsolete, they keep changing things to guarantee incompatibility. In the early days of GPS nothing had a GPS receiver in it. Now if you want GPS you can't get it without buying a new chart plotter or autopilot.
So here's my solution to the dilemma. I bought a Garmin OEM GPS receiver module for $39.95 from gpscity.com. And put it in a box. The module comes with an RS-232 interface. By adding a signal diode and a couple of resistors you can create additional EIA-422 data outputs.
Here's my stand alone GPS receiver in a box.
(http://i1121.photobucket.com/albums/l505/banjoband/garmin.png)
On the off chance that anyone finds this helpful or even interesting. Here's the schematic of the interface.
(http://i1121.photobucket.com/albums/l505/banjoband/raymarinegps-1.png)
Incidentally my chart plotter is a free Windows based package called SeaClearII. I downloaded 2600 Raster scan charts from NOAA for free also. So Raymarine can't gouge me on charts either.
Pete