D.7
Date Code 20081022 Instruction Manual SEL-787 Relay
DNP3 Communications
DNP3 in the SEL-787
acknowledged. On a large system, or in systems where the processing power
of the master is limited, you may have problems when several devices
simultaneously begin sending data and waiting for acknowledgement
messages.
The SEL-787 allows you to set the conditions for transmitting unsolicited
event data on a class-by-class basis. It also allows you to assign points to event
classes on a point-by-point basis (see Configurable Data Mapping on
page D.12). You can prioritize data transmission with these event class
features. For example, you might place high-priority points in event class 1
and set it with low thresholds (NUMEVEn and AGEEVEn settings) so that
changes to these points will be sent to the master quickly. You may then place
low priority data in event class 2 with higher thresholds.
If the SEL-787 does not receive an Application Confirm in response to
unsolicited data, it will wait for ETIMEOn seconds and then repeat the
unsolicited message. In order to prevent clogging of the network with
unsolicited data retries, the SEL-787 uses the URETRYn and UTIMEOn
settings to increase retry time when the number of retries set in URETRYn is
exceeded. After URETRYn has been exceeded, the SEL-787 pauses
UTIMEOn seconds and then transmits the unsolicited data again. Figure D.1
provides an example with URETRYn =2.
Figure D.1 Application Confirmation Timing With URETRY n = 2
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-787 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-787 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-787 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).
RX DATA
(Event Data Confirmation)
TX DATA
ETIMEOn
Retry RetryRetry
Confirmation
UTIMEOn
UTIMEOn
ETIMEOn
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