Signals

Commonly-used signals are:
• Transmitted Data (TxD)
o Data sent from DTE to DCE.
• Received Data (RxD)
o Data sent from DCE to DTE.
• Request To Send (RTS)
o Asserted (set to 0) by DTE to prepare DCE to receive data. This may require action on the part of the DCE, e.g. transmitting a carrier or reversing the direction
• Clear To Send (CTS)
o Asserted by DCE to acknowledge RTS and allow DTE to transmit.
• Data Terminal Ready (DTR)
o Asserted by DTE to indicate that it is ready to be connected. If the DCE is a modem, this may "wake up" the modem, bringing it out of a power saving mode. This behavior is seen quite often in modern PSTN and GSM modems. When this signal is de-asserted, the modem may return to its standby mode, immediately hanging up any calls in progress.
• Data Set Ready (DSR)
o Asserted by DCE to indicate an active connection. If DCE is not a modem (e.g. a null modem cable or other equipment), this signal should be permanently asserted (set to 0), possibly by a jumper to another signal.
• Data Carrier Detect (DCD)
o Asserted by DCE when a connection has been established with remote equipment.
• Ring Indicator (RI)
o Asserted by DCE when it detects a ring signal from the telephone line.
• The standard defines RTS/CTS as the signaling protocol for flow control for data transmitted from DTE to DCE. The standard has no provision for flow control in the other direction. Various implementations of compatible ports may reassign other pins for flow control.