Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

158 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
6.3.11 Flags in objects
Slave devices may ignore whether a master calls for data with or without flags, and may
send the requested data with or without a flag at the choice of the slave.
The rules are:
6.3.12 16- and 32-bit variations
• Slave may decide when to provide flag
• Slave may ignore whether master asks for flag or not
• If slave does not provide flag, point may be assumed to be normal,
ie no fault bits set
• A master must be able to process object variations with or without flags
• A slave must have a default configuration for object variation responses
to non-specific requests
• These are class or variation 0 requests
• Slave responds with default variation
• Slave must respond with specific variation if requested
6.3.13 Over-range analog objects
• If a hardware device is over-range, over-range flag is set
• But hardware data is reported unaltered
• This could be +2047 or –2048 for a 12-bit DAC for example
If a hardware device is over-range:
• Over-range bit in flag field is set
• Data is reported as is from device
If a measured value is within range of the hardware, but exceeds that which can be
represented by the data object variation requested, then a maximum positive or negative
value is returned.
6.3.14 Counter roll-over
There is a problem in interpretation of the roll-over flag for counter objects. This arises
from the flexibility provided by object variations which means a master will not know
whether a counter rolled over at 16 bits or 32 bits.
• Slaves may choose not to set the roll-over flag
• Roll-over point must be specified in device profile
• A slave must have a default configuration for object variation responses to
non-specific requests. This will specify the default counter object size
• A slave must provide at least 16 bits of a 32-bit counter if polled for 16-bit
counter object
• Master must poll frequently enough to avoid roll-over in polled systems
It is recommended that roll-over flags are not set by slave devices and are ignored by
masters.