Functional Description of the Interchange Circuit
The circuit functions are defined with reference to the DTE as follows:
Protective Ground (Shield). The protective ground ensures that the DTE and DCE chassis are at equal potential. The DCE chassis MUST NOT be tied to ground separately.
Transmitted Data (TXD). This line carries serial data from the DTE to the corresponding pin on the DCE. The line is held at a negative voltage during periods of line idle.
Received Data (RXD). This line carries serial data from the DCE to the corresponding pin on the DTE.
Request to Send (RTS). See Clear to Send (CTS) for a description.
Clear to Send (CTS). The circuit functions are defined with reference to the DTE as follow When a half duplex modem is receiving from another modem, the DTE keeps RTS inhibited. When it is the DTE's turn to transmit, it advises the modem by asserting the RTS pin. When the modem asserts the CTS, it informs the DTE that it is now safe to send data. The procedure is reversed when switching from transmit to receive.
Data Set Ready (DSR). This is also called DCE Ready. In the answer mode, the answer tone and the Data Set Ready are asserted two seconds after the telephone goes off-hook.
Signal Ground (Common). This is the common return line for the data Transmit and Receive signals. The connection between the two ends is always made.
Data Carrier Detect (DCD). This is also called the Received Line Signal Detector. It is asserted by the modem when it receives a remote carrier and remains asserted for the duration of the link.
DTE Ready. DTE Ready enables (but does not cause) the modem to switch onto the line. In originate mode, DTE Ready must be asserted for the duration of the link.
Ring Indicator. This pin is asserted during a ring on the line.
Data Signal Rate. Selector (DSRS). When two data rates are possible, the higher is selected by asserting DSRS.