Garmin GPSmap 78sc , Garmin 78

[CaptnBry]

Hello Hardware Interface Wizards,

I have Garmin GPSmap 78sc(a handheld) and the cable with the 4-pins to "OUTPUT DATA STREAM".
It can send very stable GPS and Compass data, so I want to use this on-hand device in OpenPlotter.
The firmware within the 78sc is set to output, and seems to be working, so the question is just hardware at this point, thanks

The question is: How do I get the Garmin data into my Raspberry Pi OpenPlotter/SignalK system?

I think I need an RS232 or RS422 or ?? voltage level converter of some type.

The cable also supplies 12vdc to the Garmin, so I think the voltages will be above 5vdc.

My Fluke DVM shows both positive and negative voltages, with a 'burst' type of fluctuation which I trust is Data
(yes, i need an o-scope, but mine is a victim of Ocean Aging at this point)

I expect to be soldering rtl232 chips or transistors on perf-board with an Arduino to format to JSON/Signalk to solve this

for reference, consider my 'level' as a retired EE with industrial experience unrelated to the subject herein


Thanks !