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Modem Signaling 73
Pins 2, 3, and 7 enable data transfer, pins 4 and 5 enable flow control of data, and pins 6, 8, and
20 provide modem control.
Data Transfer
The pins used for data transfer are pin 2, 3, and 7. The DTE device raises the voltage on the RTS
when it has buffer space available to receive from the DCE device. Once a call is established
and the DTE device sees the DCE raise the voltage on the CTS, the DTE device transmits data
on pin 2. Conversely, the DTE device will raise the voltage on the RTS when it has buffer space
available to receive from the DCE device. The need for the ground pin is such that a positive or
negative voltage can be discerned.
Data Flow Control
The RTS pin and the CTS pin control the flow of information. The DTE device controls the RTS
pin (as shown in Tabel 4-2), which, when seen by the DCE, alerts the DCE that it can receive
data. It might help you to think of the RTS as the ready-to-receive pin. The DCE device controls
the CTS pin, which in turn signals the DTE that it has buffer available. These definitions are
critical to a CCNP or CCDP candidate.
Modem Control
DSR and DTR are signal pins used to control how the modem operates. The DSR pin is raised
when the modem is powered on. This raising lets the DTE device know that the modem is ready
for use. The DTR pin is raised when the DTE device is powered and ready to receive
information from the DCE.
In most cases, when the DTE device is powered on, the DTR pin is raised; however, there are
cases in which the DTR pin is raised only if a software package begins to run. This might sound
like a minor point, but when you are troubleshooting, it is important to know if the DTE has
signaled the modem that it is ready. In fact, just because the PC is on does not necessarily mean
that DTR is asserted, and whether your DTE device raises the DTR when powering up or when
you turn on your communication software, DTR is needed for a two-way conversation between
the DCE and DTE device.
Note that the CD pin is also a signal pin. When two DCE devices establish a connection, the
CD pin is asserted to indicate that a carrier signal has been established between the DCE
devices. Note also that because two devices constitute the DTE (PC) and DCE (modem)
connection, either must be allowed to terminate the connection.