Tandy DCM-7 Operation Manual page 53

Appendix B
S16
Self-test toggle
The self-test mode forces the Modem to "talk to itself." Apply value 1 to
S16 when you want to set the Modem to self-test mode. Using this feature,
you will be able to tell if the Modem is functioning correctly.
When the Modem is in self-test mode, the internal switches are selected such
that data transmitted from the computer is looped back to the receiver and
processed just as if it were in communications over the telephone lines. Keys
entered from the computer are echoed to the screen.
Since the Modem goes off-hook during the test, be sure to disconnect the
wall jack phone line cord from the wall jack before executing a self-test.
To set the Modem for self-test mode, type:
AT S16=1 D
(ENTER)
Self-test mode ON.
Originate mode.
To set the Modem for self-test mode in answer mode, type:
AT S16=1 A
(ENTER)
Self-test mode ON.
Answer mode.
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