RS232 Levels

[michaelb]

Hi,

Some electronics questions. I'm using a Garmin eTrex to interface with a PIC16F877, using the hardware UART, and it's corresponding RX pin.

1) The GPS provides 4V high, and a -3V(ish) low. If Vdd is 5V and the schmitt triggers require 0.8Vdd = 4Vto go high, why does connecting the serial out of the GPS directly to the PIC RX not work?

2) I've used a 2 resistors and a NPN transistor to interface the GPS to the PIC.
5V - R1 - UART RX
R1 - Collector
0V - Emitter
GPS Out - R2
R2 - Base
As I have the PIC input connected to the collector, it is actually low when the GPS is high. i.e inverted. I saw this method online and never gave it much thought. Why does this work?

These 2 questions may well be linked - i suppose if the GPS output is inverted this would explain both.

Thanks for any help.

Michael

[UFAnders]

I imagine that a self-contained handheld module like the eTrex has standard (inverted) serial I/O. The PIC's hardware UART is TRUE LEVEL (not inverted) serial I/O. Perhaps this answers your question?

[michaelb]

Excellent - thanks for that. Didn't realize that standard was inverted. It all becomes clear now.

Thanks again,

Michael