Hardware Reference
In-Depth Information
maintains the connection. When DTR is de-asserted, the modem is switched to on-hook
to terminate the connection (same as placing the phone back on the telephone socket).
Pin 8—Received line signal detector, also called carrier detect (CD). This signal is rel-
evant when the DCE device is a modem. It is asserted (logic 0) by the modem when the
telephone line is off-hook, a connection has been established, and an answer tone is be-
ing received from the remote modem. The signal is de-asserted when no answer tone
is being received or when the answer tone is of inadequate quality to meet the local
modem's requirements.
Pin 12—Secondary received line signal detector (SCD). This signal is equivalent to the
CD (pin 8) signal but refers to the secondary channel.
Pin 22—Ring indicator (RI). This signal is relevant when the DCE device is a modem
and is asserted (logic 0) when a ringing signal is being received from the telephone line.
The assertion time of this signal is approximately equal to the duration of the ring signal,
and it is de-asserted between rings or when no ringing is present.
Pin 23—Data signal rate selector . This signal may originate in either the DTE or the
DCE devices (but not both) and is used to select one of two prearranged baud rates. The
assertion condition (logic 0) selects the higher baud rate.
T RANSMITTER AND R ECEIVER T IMING S IGNALS
This group consists of the following signals:
Pin 15—Transmitter signal element timing, also called transmitter clock (TC). This
signal is relevant only when the DCE device is a modem and is operating with a syn-
chronous protocol. The modem generates this clock signal to control exactly the rate at
which data is sent on TxD (pin 2) from the DTE device to the DCE device. The logic 1-
to-logic 0 (negative to positive transition) transition on this line causes a corresponding
transition to the next data element on the TxD line. The modem generates this signal
continuously, except when it is performing internal diagnostic functions.
Pin 17—Receiver signal element timing, also called receiver clock (RC). This signal is
similar to TC, except that it provides timing information for the DTE receiver.
Pin 24—Transmitter signal element timing, also called external transmitter clock (ETC),
with timing signals provided by the DTE device for use by a modem. This signal is used
only when TC and RC (pins 15 and 17) are not in use. The logic 1-to-logic 0 transition
indicates the time center of the data element. Timing signals will be provided whenever
the DTE is turned on regardless of other signal conditions.
C HANNEL T EST S IGNALS
This group consists of the following signals:
Pin 18—Local loopback (LL). This signal is generated by the DTE device and is used to
place the modem into a test state. When LL is asserted (logic 0, positive voltage), the mo-
dem redirects its modulated output signal, which is normally fed into the telephone line,
back into its receive circuitry. This enables data generated by the DTE to be echoed back
through the local modem to check the condition of the modem circuitry. The modem as-
serts its test mode signal on pin 25 to acknowledge that it has been placed in LL condition.
Pin 21—Remote loopback (RL). This signal is generated by the DTE device and is used
to place the remote modem into a test state. When RL is asserted (logic 0), the remote
modem redirects its received data back to its transmitted data input, thereby remodulating
 
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