Black Box MD1640A, MD1641A manual Ring Indicator Pin 22, RI CE, External Transmit Clock Pin 24, XTC

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.

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