G-2 Distributed Network Protocol (DNP) 3.00 Date Code 20010731
SEL-352-1, -2 Instruction Manual
The RTS signal may be used to control an external transceiver. The CTS signal is used as a
DCD input, indicating when the medium is in use. Transmissions are only initiated if DCD is
deasserted. When DCD drops, the next pending outgoing message may be sent once an idle time
is satisfied. This idle time is randomly selected between the minimum and maximum allowed
idle times (i.e., MAXDLY and MINDLY). In addition, the SEL-352 Relay monitors received
data and treats receipt of data as a DCD indication. This allows RTS to be looped back to CTS
in cases where the external transceiver does not support DCD. When the SEL-352 Relay
transmits a DNP message, it delays transmitting after asserting RTS by at least the time in the
PREDLY setting. After transmitting the last byte of the message, the SEL-352 Relay delays for
at least PSTDLY milliseconds before deasserting RTS. If the PSTDLY time delay is in progress
(RTS still high) following a transmission and another transmission is initiated, the SEL-352
Relay transmits the message without completing the PSTDLY delay and without any preceding
PREDLY delay. The RTS/CTS handshaking may be completely disabled by setting PREDLY to
OFF. In this case RTS is forced high and CTS is ignored, with only received characters acting as
a DCD indication. The timing is the same as above, but PREDLY functions as if it were set to 0,
and RTS is not actually deasserted after the PSTDLY time delay expires.
DATA-LINK OPERATION
It is necessary to make two important decisions about the data-link layer operation. One is how
to handle data-link confirmation; the other is how to handle data-link access. If a highly reliable
communications link exists, the data-link access can be disabled altogether, which significantly
reduces communications overhead. Otherwise, it is necessary to enable confirmation and
determine how many retries to allow and what the data-link time-out should be. The noisier the
communications channel, the more likely a message will be corrupted. Thus, the number of
retries should be set higher on noisy channels. Set the data-link time-out long enough to allow
for the worst-case response of the master plus transmission time. When the SEL-352 Relay
decides to transmit on the DNP link, it has to wait if the physical connection is in use. The
SEL-352 Relay monitors physical connections by using CTS input (treated as a Data Carrier
Detect) and monitoring character receipt. Once the physical link goes idle, as indicated by CTS
being deasserted and no characters being received, the SEL-352 Relay will wait a configurable
amount of time before beginning a transmission. This hold-off time will be a random value
between the MINDLY and MAXDLY setting values. The hold-off time is random, which
prevents multiple devices waiting to communicate on the network from continually colliding.
DATA ACCESS METHOD
Based on the capabilities of the system, it is necessary to choose a method for retrieving data on
the DNP connection. The following table summarizes the main options, listed from least to most
efficient, and indicates corresponding key related settings.
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