Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Applications of DNP3 and SCADA protocols 377
Introduction to PDS RTUs
The Hunter Watertech (HWT) PDS500, PDS Compact 500, PDS Compact 550 and
PDS 104 are intelligent microprocessor based telemetry units. They may be used as
stand-alone devices, or in conjunction with small PLC systems, for data acquisition and
telemetry applications. They are powerful and flexible in the roles they can perform. The
primary functions of the PDS family of devices are described below.
Telemetry unit
The basic role of a PDS is to operate as a telemetry unit that forms part of a network based
on the DNP3 protocol. They have the capacity to use a wide range of alternate protocols
allowing the PDS to act as a communications interface for PLC installations and other
devices. Their medium of communication can include voice and digital radio, landline,
modem and RS-232/422/485 serial connections. Comprehensive network features allow
for inter-device communications including store and forward for RTUs in locations
beyond normal range.
Programmable logic controller
The PDS500 and PDS Compact RTUs provide varying numbers of digital and analog lines
to service sites with minimal I/O to those with vast requirements. The PDS Compact 500
has 16 DIs, 8 DOs, 4 AIs and 1 AO. The PDS Compact 550 has exactly double the I/O of
the PDS Compact 500. The PDS500 on its own has 2 DOs, 2 DIs and 1 AI, but is expandable
with up to 15 plug in modules that each support up to 16 digital or 8 analog I/O lines.
The PDS 104 is used as a communications device having 5 communication channels, and
one DI (typically used for AC phase failure alarms). Other products are available in
HWT’s PDS range of RTUs.
The communications philosophy in this example system uses DNP3 protocol between
the HWT RTUs in the field and the master station, based on CI Technologies’ Citect HMI
system.
Communication philosophy
Some SCADA systems collect data by continuously polling. On fast networks, such as
PLC or Ethernet networks, the poll times for data can be set in terms of milliseconds.
When a SCADA master is communicating with remote sites via collision based radio
networks, dial-up networks, etc, transmission rates can be as low as 1200 bps. Polling may
occur at anything from 5 minutes to a number of hours, depending on how many sites are
on the network and the required data update rates. Control and monitoring at this rate of
data polling may not be acceptable for many conventional systems, and may lead to errors
when real time trending or remote control is attempted.
To avoid these problems, data is collected using DNP3 protocol at a lower network rate
while allowing access to the communication network for other purposes such as DNP3
peer-to-peer communications and RTU maintenance, remotely. PDS RTUs can be configured
to pre-empt the polling system, so that if specified, digital and/or analog points
change in value the RTU will immediately time-tag the changes and report them to the
master station. This utilizes DNP3 protocol’s unsolicited response facilities.