User manual

IEC61131 User and Reference Manual
April 22, 2008
484
Table 0-1: Hold Time and Hold Count Setup in for Six DNP Outstations
DNP Outstation
Address
Hold Time
(seconds)
Hold Count
11
1
100
12
1
100
13
1
100
14
2
100
15
2
100
16
2
100
Master not polling frequently causing event buffer overflows
An outstation does not discard the events within its buffer until all its configured masters have
acknowledged receipt of these events. This means that an outstation event buffer may eventually fill
up and overflow leading to loss of events. Buffer overflows typically indicate a poorly configured
system.
When the system is designed around unsolicited messaging, there is a good likelihood of media
contention causing buffer overflows. On the contrary, if the system is designed around frequent
master poll for event data, there will be fewer chances of buffers overflowing causing loss of event
data.
As stated earlier, immediate reporting of events using unsolicited messaging should be reserved for
those critical, yet absolutely rare occurring events. This is because unsoliciting these messages back
to the master will be reliable only if there is a substantial amount of unused bandwidth on the
communication media. A good rule of thumb is to have 50% or more of unused bandwidth available,
evenly distributed over a time frame.
Recommendation: Design the system around frequent master poll of class events and less regular
integrity polls. Reserve unsolicited messaging for infrequent high priority events. If network traffic
is predominated by unsolicited messaging, allocate 50% or more unused bandwidth as quiet time.
Outstation reports to Multiple Masters with Poor Comms
A poor communication link to one of an outstation’s multiple masters will prevent the outstation’s
event buffer to be emptied, as events cannot be reported to the master. This could lead to buffer
overflow situations and loss of event data.
Recommendation: Ensure that the communication path to all masters of an outstation is robust.
Insufficient Use of Input Deadband or Debounce
Event generation on a DNP analog input is controlled by a Deadband parameter. On a digital input,
event generation is controlled by a debounce parameter. Default settings of zero for these
parameters are typically overly aggressive and may lead to events being generated due to noise.
Recommendation: Set the analog Deadband and debounce parameters appropriately to non-zero
values.