Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

226 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
8.6.2 Command information elements
The following information elements from Table 8.15 are presented in this sub-section.
Symbol
SCO
DCO
RCO
Description
Single command
Double command
Regulating step command
Table 8.15 Extract C
Command information elements
SCO Single command
This is a command to operate a single output. Bits 2 to 7 are the Qualifier of Command
sub-field. This is used for other commands also.
7 6 5 4 3 2 1 0
QOC SCS
QOC
7 6 5 4 3 2
S/E
QU
Key – Command
SCS Single command state BS1[0]
= Command OFF
= Command ON
Key – QOC Qualifier of command
QU Qualifier UI5[2..6]
= No additional definition
= Short pulse duration
= Long duration pulse
= Persistent output
= Reserved for further standard definitions
= Reserved for selection of other predefined functions
= Reserved for special use (private range)
S/E Select/Execute BS1[7]
= Execute
= Select
An important feature of this command is that it has two forms depending on bit 7, the
select and the execute form. The select form is used when select before execute operation
is required. This is also known as two-phase command operation.