Schweitzer Engineering SEL-351P, Быстрый старт

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H-2 Distributed Network Protocol (DNP) V3.00 Date Code 991201
SEL-351P Manual Técnico
following a transmission, and another transmission is initiated, the SEL
-
351P 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.
D
ATA
-L
INK
O
PERATION
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
-
351P decides to
transmit on the DNP link, it has to wait if the physical connection is in use. The SEL
-
351P
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
-
351P 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.
D
ATA
A
CCESS
M
ETHOD
Based on the capabilities of the system, it is necessary to determine which method is desired to
retrieve data on the DNP connection. The following table summarizes the main options, listed
from least to most efficient, and corresponding key related settings are indicated.
Table APPENDIX H: .1: Data Access Methods
Data Retrieval Method Description Relevant SEL-351P Settings
Polled Static The master polls for static (Class
0) data only. Set ECLASS = 0,
Set UNSOL = N.
Polled Report-by-
Exception The master polls frequently for
event data and occasionally for
static data. Set ECLASS 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 ECLASS to a non-zero value,
Set UNSOL = Y,
Set NUMEVE and AGEEVE
according to how often messages are
desired to be sent.