I am looking at changing a circuit which needs to trigger a relay when a serial RS232 cable is connected. These need to be low powered as they are battery supplied. There are only TX, RX and GND lines available. I can see 2 avenues:

1. The TX line when not transmitting is about -6V. I was thinking of using this signal to an RC timer through a diode to power the base of a transistor.

2. Currently the circuit uses a MAX221 datasheet which seems to have an auto shutdown when an invalid signal level is detect. There is a -invalid pin on this IC and I'm interested in anyone having experience with this chip and pin?

 

I assume you are also TX/RX'ing RS232 at the same time?
You could use the MAX232 which has more I/O and parallel one of the driver inputs for the auxiliary out to trigger a relay on its output.
Max.