Hardware Reference
In-Depth Information
4. Modem status and control signals . These signals indicate modem status and provide
intermediate checkpoints as the telephone voice channel is established.
5. Transmitter and receiver timing signals . If a synchronous protocol is used, these
signals provide timing information for the transmitter and receiver, which may
operate at different baud rates.
6. Channel test signals . Before data is exchanged, the channel may be tested for its
integrity and the baud rate automatically adjusted to the maximum rate that the
channel could support.
S IGNAL G ROUND
Pins 7 and 1 and the shell are included in this category. Cables provide separate paths for
each, but internal wiring often connects pin 1 and the cable shell or shield to the signal ground
on pin 7. All signals are referenced to a common ground as defined by the voltage on pin 7. This
conductor may or may not be connected to protective ground inside the DCE device.
P RIMARY C OMMUNICATION C HANNEL
Pin 2 carries the transmit data (TxD) signal, which is active when data is transmitted from
the DTE device to the DCE device. When no data is transmitted, the signal is held in the mark
condition (logic 1, negative voltage).
Pin 3 carries the received data (RxD), which is active when the DTE device receives data
from the DCE device. When no data is received, the signal is held in the mark condition.
Pin 4 carries the request to send (RTS) signal, which is asserted (logic 0, positive voltage) to
prepare the DCE device for accepting transmitted data from the DTE device. Such preparation
might include enabling the receive circuits or setting up the channel direction in half-duplex
applications. When the DCE is ready, it acknowledges by asserting the CTS signal.
Pin 5 carries the clear to send (CTS) signal, which is asserted (logic 0) by the DCE device
to inform the DTE device that transmission may begin. RTS and CTS are commonly used as
handshaking signals to moderate the flow of data into the DCE device.
S ECONDARY C OMMUNICATION C HANNEL
Pin 14 is the secondary transmitted data (STxD). Pin 16 is the secondary received data
(SRxD). Pin 19 carries the secondary request to send (SRTS) signal. Pin 13 carries the second-
ary clear to send (SCTS) signal. These signals are equivalent to the corresponding signals in
the primary communications channel. The baud rate, however, is typically much slower in the
secondary channel, for increased reliability.
M ODEM S TATUS AND C ONTROL S IGNALS
This group includes the following signals:
Pin 6—DCE ready (DSR) . When originating from a modem, this signal is asserted (logic
0) when all the following three conditions are satisfied:
1. The modem is connected to an active telephone line that is off-hook .
2. The modem is in data mode, not voice or dialing mode.
3. The modem has completed dialing or call setup functions and is generating an answer tone.
If the line goes off-hook, a fault condition is detected, or a voice connection is estab-
lished, the DCE ready signal is de-asserted (logic 1).
Pin 20—DTE ready (DTR) . This signal is asserted (logic 0) by the DTE device when it
wishes to open a communications channel. If the DCE device is a modem, the assertion of
DTR prepares the modem to be connected to the telephone circuit and, once connected,
 
Search WWH ::




Custom Search