Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Pin 8 – Data carrier detect (DCD)
Fundamentals of SCADA communications 39
This is also called the received line signal detector. Pin 8 is asserted by the modem when
it receives a remote carrier and remains asserted for the duration of the link.
Pin 20 – DTE ready (or data terminal ready)
DTE ready enables, but does not cause, the modem to switch onto the line. In originate
mode, DTE ready must be asserted in order to auto dial. In answer mode, DTE ready must
be asserted to auto answer.
Pin 22 – Ring indicator
This pin is asserted during a ring on the line.
Pin 23 – Data signal rate selector (DSRS)
When two data rates are possible, the higher is selected by asserting pin 23.
2.7.5 The sequence of asynchronous operation of the RS-232 interface
Asynchronous operation is arguably the more common approach when using RS-232 and
will be examined in this section using the more complex half-duplex data interchange.
It should be noted that the half-duplex description is given as it encompasses that of
full-duplex operation.
Figure 2.23 gives a graphical description of the operation with the initiating user
terminal (or DTE) and its associated modem (or DCE) on the left of the diagram and the
remote computer and its modem on the right.
The following sequence of steps occur:
• The initiating user manually dials the number of the remote computer
• The receiving modem asserts the ring indicator line (RI – pin 22) in a pulsed
ON/OFF fashion as per the ringing tone. The remote computer already has its
data terminal ready line (or DTR – pin 20) asserted to indicate that it is ready
to receive calls. (Alternatively the remote computer may assert the DTR
line after a few rings.) The remote computer then sets its request to send line
(RTS – pin 4) to ON
• The receiving modem then answers the telephone and transmits a carrier
signal to the initiating end. It also asserts the DCE ready (DSR – pin 6) after
a few seconds
• The initiating modem then asserts the data carrier detect line (DCD – pin 8).
The initiating terminal asserts its DTR (if it is not already high). The modem
then responds by asserting its data set ready line (DSR – pin 6)
• The receiving modem then asserts its clear to send line (CTS – pin 5) which
permits the transfer of data from the remote computer to the initiating side
• Data is then transferred from the receiving DTE on pin 2 (transmitted data) to
the receiving modem. The receiving remote computer can then transmit
a short message to indicate to the originating terminal that it can proceed with
the data transfer. The originating modem transmits the data to the originating
terminal on pin 3