Modem control signals

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Modems are often used to initiate and receive calls. It is therefore important to program the modem to negotiate a connection at the highest possible speed and to reset itself to a known state after a connection is stopped.

The server will toggle the Data Terminal Ready (DTR) signal from on to off to instruct the modem to terminate the connection. Most modems can be configured to reset themselves when this on-to-off DTR transition occurs.

Note: The tty can be configured to not drop DTR by disabling the hupcl flag in the stty run-time attributes.

For the connection between the server and the modem to be fully functional, the cabling must have the following qualifications:

It must meet specifications.
It should be properly shielded.
The following signals should be provided: RxD, TxD, RTS, CTS, SG, DCD, and DTR.
Note: The 16-port asynchronous adapter does not provide support for the RTS and CTS signals. It is therefore impossible to use RTS/CTS hardware flow control with this adapter.

If binary data is to be transferred using a modem on this adapter, a file transfer protocol that detects incorrect data and resends the missing data (for example, Xmodem, zmodem, Kermit, and UUCP) should be used.

The following describes the signals used by the server:

Signal	Description
FG	Frame Ground. Pin 1 of the EIA 232D specification that provides for a cable shield. Properly used, the signal is attached at pin 1 on one side of the cable only and is connected to a metal sheath around the cable.
TxD	Transmit Data. Pin 2 of the EIA 232D specification. Data is transmitted on this signal. Controlled by the server.
RxD	Receive Data. Pin 3 of the EIA 232D specification. Data is received on this signal, controlled by the modem, which is sent by the modem.
RTS	Request To Send. Pin 4 of the EIA 232D specification. Used when RTS/CTS flow control is enabled. This signal is brought high when the system is ready to send data and dropped when the system wants the modem to stop sending data.
CTS	Clear To Send. Pin 5 of the EIA 232D specification. Used when RTS/CTS flow control is enabled. This signal will be brought high when the modem is ready to send or receive data. It will be dropped when the modem wishes the server to stop sending data. Controlled by the modem.
DSR	Data Set Ready. Pin 6 of the EIA 232D specification. Signals the server that the modem is in a state where it is ready for use. Controlled by the modem.
SG	Signal Ground. Pin 7 of the EIA 232D specification. This signal provides a reference voltage for the other signals.
DCD	Data Carrier Detect. Pin 8 of the EIA 232D specification. This provides a signal to the server that the modem is connected with another modem. When this signal is brought high, programs running on the server will be able to open the port. Controlled by the modem.
DTR	Data Terminal Ready. Pin 20 of the EIA 232D specification. This provides a signal to the modem that the server is on and ready to accept a connection. This signal is dropped when the server wishes the modem to drop connection to another modem. It is brought high when the port is being opened. Controlled by the server.
RI	Ring Indicate. Pin 22 of the EIA 232D specification. This provides a signal to the server that the modem is receiving a call. It is seldom used and is not needed for common operations. Controlled by the modem.