Schweitzer Engineering SEL-387-0, Instruction Manual

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G.3
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Distributed Network Protocol
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-387 decides to
transmit on the DNP link, it has to wait if the physical connection is in use.
The SEL-387 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-387 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.
Table G.2 Data Access Methods
Data Retrieval Method Description Relevant SEL-387 Relay Settings
Polled Static The master polls for static
(Class 0) data only. Set CLASS = 0, Set UNSOL = N.
Polled Report-by-Exception The master polls frequently for
event data and occasionally for static data. Set CLASS to a non-zero value,
Set UNSOL = N.
Unsolicited Report-by-Exception The slave devices send
unsolicited event data to the
master, and the master occasionally
sends integrity polls for static data. Set CLASS to a non-zero value,
Set UNSOL = Y,
Set NUMEVE and AGEEVE according to
how often messages are desired to be sent.
Quiescent The master never polls and
relies on unsolicited reports only. Set CLASS to a non-zero value,
Set UNSOL = Y,
Set NUMEVE and AGEEVE according to
how often messages are desired to be sent.