Originally Posted by Thermos
A question for you RS-232 experts...
I'm driving the GPS input on my ACK E-04 ELT from a GTN-650 RS-232 serial output - shielded Data Out and Data Ground lines with the shield tied to the connector backshell. The ACK connector has Power, Data In/Out and Airframe Ground connections.
The power and -650 Data Out to ELT Data In pins are straightforward, but is the -650 Data Ground tied to the ACK Airframe Ground? Is the line shield tied to Airframe Ground as well?
Thanks in advance for your help...
Rs-232 needs three lines to work bidirectionally:
RX - Receive
TX - Transmit
Ground - yes, this is the same ground that is tied to the aircraft.
There is also a shield that protects the line formal external rf sources. Many people use the shield for the ground. It works and on the 650 probably wonít cause any issues. Although the ground and shield serve two different functions and on anything intercom related should be treated separately to reduce noise. Having grounds connected to too many points on the airframe create whatís called ground loops. These have a tendacy to be suspectible to rf noise and it may be heard in your intercom.
If you only need unidirectional data, like the gps coordinates going to the elt, you donít need the rx line on the gps.
On my RV, I used the Tx on the 650 and the shield is attached to the ground pin on the 650. The end of the shield at the elt is not attached to anything. Itís just functioning as a shield, not a ground. As long as both units have a path to ground, the circuit will work. All grounds are usually tied together within a device. You can use an ohm meter to validate.