For the past 15 years, the widely used DNP3 protocol has worked well as a means of communicating data over slow and intermittent links. But it does have a number of shortcomings. To combat this, a quiet revolution is underway in the world of remote terminal units (RTUs).
The DNP3 protocol is a point-to-point master/slave protocol, not a peer-to-peer protocol, but does allow some peer-to-peer by allowing the slave to emulate a master under certain circumstances. It allows the RTU to communicate with the HMI, but peer-to-peer communications are limited, and RTU polling must be a special application written for a client master station.
Yokogawa — better known for its extensive experience in distributed control systems (DCS) — has taken the peer-to-peer token passing network approach of its DCS and applied it to RTU applications.
The ‘holy grail’ is a network protocol that provides deterministic, peer-to-peer communications for low bandwidth, intermittent communication links for RTUs, while still maintaining the advantages of DNP3. It must work over a variety of communication carriers, such as radio, internet, PSTN, GSM, and GPRS, in a transparent manner.
Yokogawa has developed just such a network, and overcame a number of challenges in doing so. For example, the Vnet (the Yokogawa token passing LAN) model could not be applied directly because of the nature of RTU communications.
A single token being passed around each device would not allow alarms and other high priority data to be reported quickly enough.
Innovative thinking about how tokens work in a network environment resulted in a solution. In addition, the logging capability of the RTU has been employed to provide periodic, by-exception data that is transmitted at a lower rate. This rate can be as fast as the network allows, and as slow as a one year cycle, due to the large storage capacity of the Yokogawa RTU.
One of the most important design criteria was the peer-to-peer functionality of the protocol. As with Vnet, there is no master that controls the network. All devices can communicate with each other, and an HMI is just another node on the network. And like Foundation Fieldbus, device and network information is readily available for system diagnostics.
The networking protocol allows signals to be routed across different networks. This makes it possible to have independent networks (domains) with their own communication carriers (radio, internet, PSTN, GSM and the like) with RTUs communicating across these networks.
An important consideration in designing the protocol was that it was transparent to the user. Any HMI with OPC support needs to be able to connect into the network without knowing anything about the communication process. The Yokogawa solution means that the HMI simply requests live data through OPC-DA, alarm and events through OPC-AE and historical data through OPC-HDA. That is, this new RTU network protocol is a true network protocol rather than just a communications protocol.
It provides: peer-to-peer communications; deterministic token passing networking; alarm and event message handling; logging of data for long periods in the event of loss of communications; routing across networks; support for OPC; network time synchronisation; independence from the communications carrier; and a design suitable for low bandwidth, intermittent networks.
Yokogawa is currently implementing the protocol on a customer site in Christchurch, New Zealand, where 300 controllers are being progressively installed and will be communicating by radio using this innovative network protocol. Discussions are also underway with customers in the water/waste-water, oil and gas and mining sectors, where this breakthrough technology is most likely to deliver significant benefits.
• David Walker
Yokogawa Australia
www.yokogawa.com/au
inquiries@au.yokogawa.com