Open COM Serial Port

Technical Programming
1

Hey all,

I have a Garmin Etrex H which streams serial data in rs232 format. It is simply ASCII characters. A terminal program on my computer receives them and displays them on my screen just fine. Also, my RC is able to send and receive data to and from my terminal program. Both the RC and the Garmin are set to 9600 baud, no parity, and one stop bit.

However, when I attach the Garmin directly into the RC, the RC does not receive any data. I think this is because the RC is waiting for something to start communication with it, and the Garmin is also waiting. How can I open port 1 on the RC and start communication with the Garmin? I’m using Kevin Watson’s serial code.

Thanks alot!

2

You need to do a level shift of RS232 voltages to TTL voltages. Are you sure you have Transmit on the GPS going to the Receive Pin?

When I hook the GPS to the Vex controller, I use the orange level shifter and a null modem to get the signals to swap.

3

I’m plugging it in to the program port, which I believe is already rs232. It shouldn’t need a level shifter.

4

I set the baud rate of port 1 (the program port ) to 9600 using Kevin’s code. And since it can talk to the terminal program on my computer at baud 9600, i know its working at that baud.

5

Sounds like you might need a null modem

6

Ya some of the things on that Wikipedia sound like it would fix the problem.

Why though would swapping the Tx and Rx lines make the Garmin send data to the RC all of a sudden?

Thanks

7

The null modem worked! Thank a lot!

8

The wiring was such that Tx was connected to Tx, and Rx to Rx. The adapter simply swapped it such that Tx->Rx, and Rx<-Tx. Voila!

Lovely little things to have around, especially when you make your own cable harnesses.

9

Thus, if I’m not mistaken, the magic behind the Red Tether Cable (simply a null modem cable).

10

Bingo :wink: