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APPENDIX I: RS232C Interface Specifications
turned off at the conclusion of the log off procedure, which forces the
modem to disconnect, enabling it to receive another call. In non auto
answer applications it is advisable to leave DTR on using the modem's
DIP-switch option. This is the standard factory setting. An alternative is
to provide a constant high DTR from the terminal or computer. The
Data Terminal Ready (DTR) LED indicates the status of this signal.
Ring Indicator - Pin 22, RI (CE)
Direction: from modem
This signal will remain on for the duration of the ringing signal. When a
ring signal is received by the modem, the modem will automatically
answer after the first ring. The modem will answer after a specified
number of rings which can be programmed in AT Command mode, but
if none is specified, the modem will answer after the first ring.
External Transmit Clock - Pin 24, XTC
Direction: to modem
Supplies the same function as transmit clock on Pin 15.
Terminal Busy (Out of Service) - Pin 25, OOS
Direction: to modem
The Terminal Busy (OOS) circuit is not defined in the RS232C
standard, but is used by us and most manufacturers to make the phone
lines busy to incoming phone calls. Pin 25 is not normally connected to
the RS232C 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 will be placed Off Hook, and busy to incoming calls.