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Appendix
Ring Indicator (RI) — Pin 22 (From modem)
This signal remains on for the duration of the ringing signal. When a ring signal is received
by the modem, the modem automatically answers after the first ring. The modem can
answer after a specified number of rings which can be programmed in command mode, but
if none is specified, the modem answers after the first ring.
External Transmit Clock (XTC) — Pin 24 (To modem)
Supplies the same function as the Transmit Clock signal on pin 15.
Terminal Busy (OOS) — Pin 25 (To modem)
The Terminal Busy/Out of Service (OOS) circuit is not defined in the RS-232C standard, but
is used by Multi-Tech Systems and most manufacturers to make the phone lines busy to
incoming phone calls. Pin 25 is not typically connected to the RS-232C interface when
shipped from the factory; it can be ordered connected as an option. If pin 25 is connected
and is brought high, the modem is placed off hook, and is busy to incoming calls.