RS232 Communications with a 5V GPS Receiver

[weg22]

Hi all,

I have a Garmin GPS receiver (GPS 18 5Hz) which is powered with +5V and uses RS232 to transmit GPS data. I designed a circuit which includes a PIC16F877 microcontroller and a Maxim RS232 chip (MAX232). I am using the MAX232 chip as the middleman...both the Tx and Rx lines of the PIC and GPS receiver are fed into the MAX232 chip. Everything works, but my question is do I really need the MAX232 chip since the GPS receiver is powered with +5V and not +12V? Will my setup be more efficient if I just eliminate the MAX232 chip and just feed the Tx and Rx lines of the GPS receiver directly into the Tx and Rx lines of the PIC?

Thanks in advance,
-weg

[Douglas Kennedy]

GPS receivers will vary. The max2332 5v to 12v RS232 does more that just the level transition it also provides a level of elecrotstatic pulse protection. A gps receiver will most often have its own RS232 ( max232) chip installed.
Check the voltage levels of your receiver at Tx to ground it will probably be in the 12 volt range. There is no speed advantage to 5v or 12v.
Note RS232 at 5v TTL level is inverted from the 12v level.

[GDetienne]

With the Garmin 18, the use of the Max 232 is not necessary. It's TTL level.
But two conditions : your "#use RS232(....)" must include "invert" and you don't use the internal UART.
I use it in these conditions, and all are OK.
Have a nice week end.