H-2 Distributed Network Protocol (DNP) V3.00 Date Code 20001006
SEL-351 Instruction Manual
delays transmitting after asserting RTS by at least the time in the PREDLY setting. After
transmitting the last byte of the message, the SEL-351 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-351 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-351 decides to transmit
on the DNP link, it has to wait if the physical connection is in use. The SEL-351 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-351 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 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 H.1: Data Access Methods
Data Retrieval Method Description Relevant SEL-351 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.
Courtesy of NationalSwitchgear.com
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