
Date Code 20020215
Distributed Network Protocol (DNP) V3.00
H-3
SEL-351R Instruction Manual
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.
A
UTOMATIC
D
IAL
-O
UT
To support automatic dial-out via DNP, Relay Word bits DDATA and DCONN are available for
use in SEL
OGIC
control equations. When unsolicited event data is available, DNP asserts the
DDATA Relay Word bit. If setting UNSOL = Y or DIAL and a connection has been made, the
SEL-351R begins transmitting the unsolicited event data and asserts Relay word bit DCONN. If
setting UNSOL = DIAL, the SEL-351R further checks to see that the CTS serial port line is
asserted (e.g., from an external modem), before beginning the transmission of the unsolicited
event data.
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
-
351R decides
to transmit on the DNP link, it has to wait if the physical connection is in use. The SEL
-
351R
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
-
351R 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.
Содержание SEL-351R
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