Specifications

Rockwell Automation Publication AG-SG001G-EN-P - April 2015 35
Choosing a Telemetry Network Chapter 2
DNP3
DNP3 Slave is a serial and Ethernet SCADA/RTU protocol built into the
DataSite RTU and MicroLogix 1400 controller. DNP3 Master OPC drivers for
FactoryTalk View SE software are available from Encompass Partners Kepware
Technologies and MatrikonOPC, and DNP3 Master/Slave interface modules for
various Allen-Bradley controllers are available from Encompass Partner Prosoft
Te c h n o l o g y .
Distributed Network Protocol version 3.0 (DNP3) was developed by Westronics,
an electric utility RTU manufacturer in the early '90s, specifically for SCADA/
RTU communication. They turned the specifications over to the newly formed
DNP Users Group in the mid '90s and since then this open protocol has been
implemented in hundreds of devices. While originally primarily used in the
Electric Utility Industry, it has more recently been embraced for Oil and Gas and
Water/Wastewater SCADA/RTU applications as well because it is efficient,
reliable, robust, and secure.
• Efficiency
DNP3 is an event-driven protocol. You can configure the slaves to only
report changes, like bit changes or analog inputs outside of a configured
deadband. These changes can be reported back to the master either in
response to the master's poll or as unsolicited responses generated by the
controller. There are mechanisms built in to send multiple events together
and to prevent unsolicited response ‘event storms’. The master can
periodically send an ‘integrity’ poll, which triggers the controller to send
the current value of all points in its DNP3 database, packed into as few
packets as possible. Also, over Ethernet, DNP3 gives you the flexibility of
using either TCP or UDP as its transport protocol. UDP has much lower
packet overhead and therefore is much more efficient than TCP, making it
ideal for a cellular or low bandwidth Ethernet radio SCADA system.
• Reliability
DNP3 is a SCADA protocol designed ‘from the ground up’ to work over
inherently unreliable connections. As such, DNP3 has data logging built-
in. When events are generated, they get logged into a queue, even if it's just
for a few seconds. If the master can't get a poll through for hours, or if the
unsolicited responses can't get through to the master, the events simply
continue to get added to the queue until communication is restored. The
MicroLogix 1400 can store over 6000 events in its queue.
• Robustness
With DNP3, all configured data changes are captured and reported as
events, so nothing is missed regardless of the frequency or duration of
changes. Also, all events are time-stamped at the RTU level, and all RTU
clocks are synchronized over the DNP3 network. Once the event data has
been received by the master, the data change is reported to the HMI or
historian application using the RTU millisecond timestamp. If the master
receives multiple changes for the same point, all of those changes will be
recorded with their original timestamps.