wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

DNP3 and IEC 60870-5 58-7
may be sparsely assigned anywhere in the allowed range (which can vary depending on systemwide
parameters that specify the IOA size and how it is formatted). DNP3 models all data within
a device as a set of one-dimensional arrays with a separate array for each type of data. Each data
element is identified in its array by an index number, starting at zero for the first element. In each
protocol, maximum message reporting efficiency is achieved if the IOA range or index number
range for any single data type is a single contiguous series with no gaps. For T101/T104, this
means assigning IOA values X to X + n − 1 for n objects of the same type and for DNP3 it means
assigning indices 0 to n − 1 for n objects of the same type. In this regard, the IEC data model
somewhat resembles the register mapping concept used in Modbus.
• The identification of data in a T101 or T104 system is by the unique combination of the device
identity (called the common address of ASDU or CAA) and IOA of each data element in the
device. This identification is independent of the data link address (in T101) or the IP address (in
T104) of the device reporting the data. Each different data object in a T101/T104 device (the device
having one CAA value) must have a different IOA. In DNP3, the combination of the serial channel
(or IP address) of the device, the DNP address of the device, the data type, and the object index
together uniquely identify the data object.
• Because the IEC messages can only report a single kind of data or function in a single message, the
response to some application functions consist of a series of messages starting with a “begin command
sequence” or activation confirmation message (ACTCON) followed by one or more data
messages and terminating with an “end command sequence” or activation termination message
(ACTTERM). In DNP3, the same functionality typically only requires a single response message.
• The T101 and T104 messages include a “cause of transmission” (COT) value that serves to assist
with the control of the multiple steps of the response to a command and is partly useful for indicating
why data is being sent. For example, the COT can differentiate between a change that has
occurred spontaneously (e.g., circuit breaker trip due to protection action) or because an operator
has issued a control command to cause the change. This information can be added to the relevant
event log entries. DNP3 does not include this kind of information and does not indicate why a
change occurred, merely that a change has occurred.
• DNP3 defines generic SCADA data objects (e.g., binary input, setpoint output) without assigning
specific meaning to those objects. In addition to these basic types, T101 and T104 define
a number of power-system-specific objects such as “step position information” representing a
transformer tap number, protection events, packed protection start events, and packed protection
circuit information.
• DNP3 defines some complex generic data objects such as “Octet String” objects and “Data Set” objects
that can be defined to provide functions such as multi-byte variable values or complex data structures.
DNP3 also supports reporting of device data attributes (nameplate information). T101 and T104 support
a generic 32 bit bitstring object whose purpose is left open for implementers to define.
• T101 and T104 define “normalized,” “scaled,” and “floating-point” data objects as different kinds
of analog objects. Each of the three types also has separate setpoint command types. Normalized
and scaled are both 16 bit 2’s complement integers and floating-point uses the 32 bit “short” format
defined in IEEE 754. For the integer formats, normalized data represents the values −1.0 to +1.0–2 −15
while scaled data represents the values −32768 to +32767. Both must be “scaled” before transmission
and after reception, but by different scaling factors. There is an implication in the standard that the
scaling factors used for the scaled values are always a factor of 10 (e.g., 0.001, 0.01, 0.1, 1, 10, 100, etc.)
and this simplistic approach seems to be commonly used. Devices typically only support either the
normalized or the scaled format, leading to some interoperability issues. The normalized format
seems to be slightly more commonly used than the scaled format. DNP3 treats 16 bit integer, 32 bit
integer, 32 bit floating-point, and 64 bit floating-point as alternate message formats for reporting the
same object. The format that is used should be chosen to suit the data. The integer formats can be
scaled as needed, with linear scaling (Y = AX + B) being common.
© 2011 by Taylor and Francis Group, LLC