switching output when RS232 transmits

Hi, I am looking to interrupt a RS232 signal from a weigh scale head and switch a digital output when the scale transmits data to a PC. I am using a UNO with a RS-232 to ttl shield, Thanks

interrupt a RS232 signal

What does this mean?

Verify that the scale uses RS-232 levels, not TTL.
Verify that the shield output pin is not used otherwise.
Connect the scale TX to the shield RX.
Watch the shield output pin for changes.

You might be better reading the data in and retransmitting it onwards to your PC. Then write some code to operate the digital when coms is active

Retransmission requires 2 UARTS or a SoftwareSerial.