mikem wrote:
While checking my new Garmin 496 power/data cable, I put a scope on the
NEMA DATAOut line. Just like the EI instruments, the Garmin puts out
TTL (0V and 4V) logic levels instead of the normal RS232 voltage levels
(-5V / -12V to +5V /+12V)

I had a gadget (Caller ID interface) that was designed to plug into the
serial port of a PC. It didn't have an onboard way to generate the
negative voltage - no dual power supply, nor a MAX232 or similar chip.
Instead, it depended (IIRC) on one of the handshake lines on the PC
being set to mark (negative voltage), and used a transistor to switch
this voltage onto the data line when a mark was needed. I think you
could even use the transmit line from the PC as the source of the
negative voltage (it idles in mark), as long as you could guarantee that
the software on the PC wouldn't try to talk and listen at the same time -
in other words, half duplex.

If you have an RS232 breakout box, or a cable you don't mind cutting up,
you might connect the Garmin to your laptop through the box/cable and see
what the voltage levels look like then.

Matt Roberds