Sel 751A, Инструкция по эксплуатации

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D.8
SEL-751A Relay Instruction Manual Date Code 20100129
DNP3 Communications
DNP3 in the SEL-751A
Collision Avoidance
If your application uses unsolicited reporting on a serial network, you must
select a half-duplex medium or a medium that includes carrier detection to
avoid data collisions. EIA-485 two-wire networks are half-duplex. EIA-485
four-wire networks do not provide carrier detection, while EIA-232 systems
can support carrier detection. DNP3 LAN/WAN uses features of the IP suite
for collision avoidance, so does not require these settings.
The SEL-751A uses Application Confirmation messages to guarantee delivery
of unsolicited event data before erasing the local event data buffer. Data
collisions are typically resolved when messages are repeated until confirmed.
The SEL-751A pauses for a random delay between the settings MAXDLY and
MINDLY when it detects a carrier through data on the receive line or the CTS
pin. For example, if you use the settings of 0.10 seconds for MAXDLY and
0.05 seconds for MINDLY, the SEL-751A will insert a random delay of 50 to
100 ms (milliseconds) between the end of carrier detection and the start of
data transmission (see
Figure D.2 ).
Figure D.2 Message Transmission Timing
Transmission Control
If you use a media transceiver (for example, EIA-232 to EIA-485) or a radio
system for your DNP3 network, you may need to adjust data transmission
properties. Use the PREDLY and POSTDLY settings to provide a delay
between RTS signal control and data transmission (see
Figure D.2 ). For
example, an EIA-485 transceiver typically requires 10 to 20 ms to change
from receive to transmit. If you set the pre-delay to 30 ms, you will avoid data
loss resulting from data transmission beginning at the same time as RTS signal
assertion.
Event Data
DNP3 event data objects contain change-of-state and time-stamp information
that the SEL-751A collects and stores in a buffer. Points assigned in the
Binary Input Map that are also assigned in the Sequential Events Recorder
(SER) settings carry the time stamp of actual occurrence. Binary input points
not assigned in the SER settings will carry a time stamp based on the DNP
map scan time. This may be significantly delayed from when the original
source changed and should not be used for sequence-of-events determination.
The DNP map is scanned approximately once per second to generate events.
You can configure the SEL-751A to either report the data without a polling
request from the master (unsolicited data) or hold the data until the master
requests it with an event poll message.
With the event class settings ECLASSB n , ECLASSC n , and ECLASSA n , you
can set the event class for binary, counter, analog and virtual terminal for
session
n . You can use the classes as a simple priority system for collecting
event data. The SEL-751A does not treat data of different classes differently
with respect to message scanning, but it does allow the master to perform
independent class polls.
Receipt of RX
DATA or CTS
Random Delay
MINDLY to MAXDLY
RTS
TX DATA
PREDLY PSTDLY
NOTE:
PREDLY and POSTDLY
settings are only available for EIA-232
and EIA-485 serial port sessions.