RS232 logic

bcs99 · Joined: 22 Nov 2003 Posts: 32

I'm using a PIC18F452 to communicate with a serial device using an RS232 convertor (ST232N). I'm not having any problems communicating (TX/RX) with the device The problem is that it uses the DTR line to indicate an external condition by sending a low (RS232 logic) which is the natural state of the RS232 convertor. So what I get is a low state even if the device is not connected. I need to know that the low state is valid. How do I get the RS232 logic to default to a high state?

Bob

Haplo · Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia

Connect the DTR line to one of the RS232 inputs of the ST232n (pin 8 or 13). The output will be a proper TTL level signal which is going to be a 1 when the device is not ready or disconnected.

solution · Guest

Turns out I already had the solution but was using too weak a value for a pull-up resistor. I put a 5k (tried 10k) pull-up to the +10v output and it now defaults to a high instead of a low and still allows the device to pull it low. The internal pull-down in the ST232N is also a 5k and is connected to ground.

Bob