Datamax Prodigy Operator's Manual page 39

If you want to perform the TXD/RXD and CTS/DTR loop checks, you
will need to fabricate a test plug as illustrated below in Figure 3.8
Connect the plug to the RS-232 connector on the back of the Printer and
run the self test again.
Figure 3-8 Test Plugs
If the Printer passes the loop back checks the following two line will
print on the Self Test label.
SERIAL LOOP BACK CHECK IS GOOD SERIAL LOOP BACK CHECK IS GOOD
CTS & DTR LOOP BACK CHECK IS GOOD CTS & DTR LOOP BACK CHECK IS GOOD
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Setup and Self Test Page - 31