Hi Al,

"Al Lorona" wrote in message
...
The GPS puts out +/- 5 V serial, but the Tiny Trak's serial input is a
TTL-level input to the PIC microprocessor. The PIC runs off of a single +5V
supply. I guess it can withstand the negative voltage swing without failing?

You'd have to look at the PIC's data sheet, but it's very common for digital
ICs to have appropriately connected diodes to Vcc and Ground for the sake of
ESD protection. It's not the most elegant design, but it's not uncommon to
use these built-in diodes to limit (clamp) input voltages to a diode drop
above Vcc or below ground, which -- assuming the current through the diodes is
limited, hence the power dissipated by them is -- is usually harmless.

And... if I looked at the serial output from the GPS into an open circuit,
there was no signal. It was as if the GPS shut off its serial if it detected
an open circuit. Is this normal?

I am a little surprised, to tell you the truth, but given that the Garmin is
typically battery powered, it would make sense for them to shut down the
serial interface circuitry if they don't detect anyone "listening."

---Joel