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Control Signals

The Relationship Between The RS-232 Control Signals

When the DTE becomes operational it notifies the DCE that it is ready by asserting the DTR line on Pin 20. When the DCE receives the DTR signal (on Pin 20) it responds by asserting the DSR line on Pin 6. These might be a computer and a modem saying, "The terminal is ready and, therefore, the modem is now ready".

When a modem is used the DTR signal may be used to indicate that the modem is off-hook and ready to dial. If a modem is in the autoanswer mode then DTR may be used to signal the modem to answer the phone (in response to receiving a Ring Indication on Pin 22). The modem is able to recognize that the phone is ringing. When the phone rings, RI is asserted from the modem to the computer. In response the computer asserts DTR which causes the modem to go off-hook and answer the call.

Pin 8 is the Carrier Detect line. The modem asserts the CD line when it detects a valid synchronization tone between two modems. The qualities that constitute ‘valid’ are related to the voltage levels detected on the phone line. These may be configurable either in software or hardware (modem software). The CD signal is asserted by the modem after it ‘synchs up’ with the modem on the other end. Often, when no modem is present, Pin 8 is tied directly to Pin 20 (DTR) to satisfy applications that will not transmit data unless CD is asserted.

Pins 15 and 17 are used when a modem needs to synchronize its clock with that of the DTE. In the ‘typical’ modem application these lines are not used. If an RS-232 implementation is considered to be ‘synchronous’ then the timing lines will be required.

When the DTE wants to send data it asserts the RTS line on Pin 4. If the DCE can accept the data it responds by asserting the CTS line on Pin 5. Here is the playwright’s script for the scene where the modem and the computer are conversing.

Below is a graphic representation of the steps that occur when DTE and DCE communicate. You can see that DTR is first asserted, the DSR. Next, RTS is asserted and then CTS. Finally the data flows across the circuit.