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.