I.4
SEL-311C Relay Instruction Manual Date Code 20060320
DNP Communications
Data-Link Operation
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-311C decides to
transmit on the DNP link, it has to wait if the physical connection is in use.
The SEL-311C 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-311C 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.
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