A continuous string of alternating R and Y characters transmitted over a teletype circuit to test alignment and signal quality. In the Baudot code used by early teleprinters, the binary patterns for R and Y are exact opposites (01010 and 10101). Sending them repeatedly exercises all the mechanical and electrical components of the system by forcing maximum state changes.
Heard on the job
Run an RY test loop for five minutes to see if the mechanical relays are sticking.