Schweitzer Engineering Laboratories SEL-311L, Instruction Manual, Page: 702 / 734

H-2 Distributed Network Protocol (DNP) 3.00 Level 2 Date Code 20010625
SEL-311L Instruction Manual
Relay transmits a DNP message, it delays transmitting after asserting RTS by at least the time in
the PREDLY setting. After transmitting the last byte of the message, the
SEL-
311L Relay 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-
311L Relay 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 was 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-
311L Relay
decides to transmit on the DNP link, it has to wait if the physical connection is in use. The
SEL-
311L Relay 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-
311L Relay 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 H.1: Data Access Methods
Data Retrieval Method Description Relevant SEL-311L 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.