Serial Port Control

The modems.com 28.8(V.34) / 14.4 Kbps Data/FAX Modems can determine the speed, parity, and stop bits from the serial port connection. The modem automatically detects the serial data speed between 300 and 115,200 bps with the following formats:

                             
Data Length	Parity	Stop Bits      Total Length

     7		None         2	            10
     7        	Odd          1     	    10
     7        	Even         1              10
     7        	Mark         1              10
     7        	Space        1              10
     8        	None         1              10
     8        	Odd          1              11
     8        	Even         1              11

The modem also has the capability of automatically adjusting the baud rate to the internal serial port to physical carrier speed. The user application must adjust the baud rate of the internal serial port to it by detecting carrier speed after CONNECT xxxx message. This command setting is valid for reliable (error corrected) link and normal mode connections. The baud rate adjust feature is always active for direct mode connection.

This command controls the behavior of the DCD (Data Carrier Detect) signal of the internal serial port.

• &C0 -- DCD is always on.
• &C1 -- DCD will track the state of the data carrier from the remote modem.

This command controls the modem response to the serial port's Data Terminal Ready (DTR) signal. This command interprets the ON and OFF transitions of the DTR signal from the DTE in accordance with the parameters as defined by n.

This command determines how the modem controls data flow between the system and the modem. For example, if the speed between the system unit and the modem is 19200 bps, and the speed between the local and remote modem is 2400 BPS, the transmit buffer of the modem is likely to be overflow when the system is sending large amounts of data. To prevent an overflow and subsequent loss of data, the modem either sends a XOFF character or drops the CTS signal to signal that the system should stop data sending. When enough space is available in the modems transmit buffer, the modem either sends a XON character or raises CTS to signal that application could resume data sending.

For binary data transfer CTS/RTS flow control should be selected since the modem cannot distinguish the user's data from XON/XOFF characters. Flow control is not used for direct mode connection.

• &K0 -- Disables flow control.
• &K1 -- Error
• &K2 -- Error
• &K3 -- Enables RTS/CTS flow control. Flow control is active in both sending and receiving direction.
• &K4 -- Enables XON/XOFF flow control. Flow control is active in both sending and receiving direction.
• &K5 -- Enables transparent XON/XOFF flow control. XON/XOFF character which is sent from the system unit will be processed for flow control and also sent to the remote modem. Flow control is active in both sending and receiving direction.
• &K6 -- Enables both RTS/CTS and XON/XOFF flow control.

Determines the DTR operating mode. The modem treats the &M command as a subset of the &Q command. This command is used to choose between asynchronous, synchronous, or direct dial modes while the modem is in the command state. Only Asynchronous mode is supported., and between synchronous and asynchronous data flow when a connection is established.

• &M0 -- Selects direct asynchronous mode.

This command determines how the modem controls CTS (Clear to Send). CTS operation is modified if hardware flow control is selected.

• &R0 -- In synchronous mode, CTS tracks the state of RTS. In asynchronous mode, CTS acts according to V.25 handshake.
• &R1 -- In synchronous mode, CTS is always ON (RTS transitions are ignored). In asynchronous mode, CTS will only drop if required by flow control.

Data Set Ready Control

• &S0 -- Causes DSR signal to be active at all times.
• &S1-- Causes DSR signal to be active according to the V.25 protocol.

Selects the source of the transmission clock for the synchronous mode of operation. The parameter value, if valid, is written to S27 (bits 4 and 5). In asynchronous mode, the transmission and receipt clocks are turned off.

• &X0 -- Selects internal timing.
• &X1 -- Selects external timing.
• &X2 -- Selects slave receiving timing. The transmission clock signal is derived from the incoming carrier.

This command enables and/or disables software flow control using XON/XOFF, between your modem and the remote modem. During a reliable (error corrected)reliable connection this setting is ignored.

• \G0 -- Disables modem port XON/XOFF flow control
• \G1 -- Enables modem port XON/XOFF flow control

This command determines the split-speed DCE/DTE interface for applications such as ViewData terminals, which require a transmit speed of 75 bps and a receive speed of 1200 bps at the DTE interface.

• \W0 -- Disables split-speed mode.
• \W1-- Enables split-speed mode.

This command determines which direction, transmit or receive, has the 75 bps channel and which has the 1200 bps channel. This command is only valid if the "Split-Speed Operation" command has been executed.

• %F1 -- Selects 75 tx / 1200 rx
• %F2 -- Selects 1200 tx / 75 rx
• %F3 -- Selects V.23 half duplex operation