ETHERNET for Scada?

Networking companies are pouring millions into Ethernet product development, making it a highly attractive open connectivity standard for industrial and process control. Scada (supervisory control and data acquisition) and DCS (distributed control systems) vendors may support their favourite field-device network, such as Modbus or Foundation Fieldbus, but practically every vendor supports Ethernet, enabling control engineers to take advantage of the best commercial developments as they become available.

Ethernet infrastructure is inexpensive and well understood by network specialists, who find it one of the simplest networks to implement. It now dominates the commercial sector. When the corporate strategy is to integrate information from the factory floor up through the company to board level, the immediate availability of a proven Ethernet backbone looks very attractive to the control engineer. In the long run, this may well kill off most proprietary buses in Scada systems.

The lowest level

Despite its apparent advantages, Ethernet still has a few hurdles to overcome before it becomes accepted as the universal Scada network of choice. It is important to understand that Ethernet is strictly only the very lowest layer of a network: the physical link (cable and connectors) and data link (physical addressing, framing, error checking, etc). Ethernet can't help a measurement value find its way through a complex network, nor does it define how to upload a new measurement configuration to a remote terminal or read back a set of results.

A long-standing argument against Ethernet is its non-deterministic nature - the protocol says that if the network is busy, then a node with a message, no matter how important, must wait. To understand why this might be a problem, imagine a computer implementing a PID (proportional-integral-derivative) algorithm using data collected from an RTU over an Ethernet segment. In theory, Ethernet can't guarantee consistent timing in order to tune the PID loop.

In reality, the determinism issue has been a bit of a red herring. Studies conducted in the late 1980s showed that, in practice, Ethernet delays are linear and can be consistently maintained under 30 milliseconds, even for a heavily loaded network. Such delays are inconsequential for most process control applications.

To become a workable Scada networking medium, additional protocols must be layered on top of Ethernet. Many engineers think Ethernet and TCP/IP are synonymous. In fact they are separate protocols that work well together. TCP (transmission control protocol) and IP (Internet protocol) provide routing and end-to-end data integrity. TCP/IP usually comes bundled with other application layer protocols - for example file transfer and mail transport - but these are designed for the Internet, not for a pressure transmitter or an RTU.

Fieldbus in agreement

Last year, the Fieldbus Foundation agreed to use fast Ethernet as the base-level protocol. At the base level in the hierarchy of plant networks, Fieldbus serves as a LAN (local area network) for instruments used in control and automation applications. The Foundation is now mapping all upper level protocols into Ethernet.

One of the most significant Scada implementation costs is cabling from the sensors to the measurement system. For example, thermocouple cable costs between £2 and £20 per metre, depending on the type of thermocouple; double this if 4-wire links are installed to eliminate signal losses in long runs. It was with these considerations in mind that Solartron first developed its IMP (isolated measurement pod).

The concept is extremely simple: control parameters are measured at source by accurate, high-precision data acquisition modules (IMPs). Built-in sensor energisation, signal conditioning and measurement linearisation are under the control of an internal microprocessor, enabling IMPs to provide a totally integrated front-end data acquisition capability.

With ruggedised IP55 enclosures they can be installed in harsh environments - including areas of high electrical interference and vibration - very close to the point of measurement, eliminating costly signal cable runs.

Like most Scada systems of the time, IMPs were developed before open networking was commonplace and hence Solartron developed its own proprietary network. IMPs are daisy-chained together with a low cost 2-wire network which provides power and carries command and measurement data at 163 kBit/s.

With the introduction of its new E-IMP, Solartron has extended the IMP concept with the integration of Ethernet connectivity, allowing direct connection to computers or off-the-shelf 10base-T hubs, further reducing the cost of installation.

In the past, Modbus has also proven popular among Scada vendors. Modbus is a simple open protocol used by many as the lowest common denominator to providing rudimentary communications. It is slow compared to Ethernet but robust and reliable.

Fieldbus? Ethernet? Modbus? Vendor-specific? Although cost is driving the trend towards Ethernet, there is no clear-cut networking winner. Each has its advantages and disadvantages, its advocates and detractors. Process control equipment suppliers such as Solartron must be sensitive to the developing needs of its customers, balance the trade-offs and offer a variety of solutions.

Dr Mike Page is research and development manager at Solartron.