Hi,

When you are using a PC to PIC comm. you are necessarily using a software at the PC side. In its simple form there could be two situations :

1. Your PC side program is running.

2. Your PC side program is not running.

Situation1 : Your com port is open and cannot be used by any other prog. So no issues on the TX part.

Situation2 : Your PC app for the PIC is not running and you want to use another prog with another connection.So a TX from the other devices cause a conflict with your TX output.

Using the DTR is not a recommended option as there can usage of it by the other device.

At this moment what I can think of is a thru connection via your PIC module.

Use a Relay or CMOS switch to select one of the TX (either the PIC or the shared device). Depending on the priority you want you can sense activity on the other TX and switch from the PIC to it. Otherway round is to poll for a particular string on the RX line to enable the PIC TX or it is a thru of the other device.

Please ellaborate your PC-PIC and other device connections. Whether you are using a level translator or a simple resistor. Also which PIC you are using and the type of other device (PIC type or true RS232)

Regards

Sougata