Author: Lester Abbey

Definitions

An Open System is a system where the major components adhere to certain standards. These major components can be interchanged with similar components manufactured by others to the same standards. An IBM compatible PC is an example of an Open system.

An Open System is a system where the major components adhere to certain standards. These major components can be interchanged with similar components manufactured by others to the same standards. An IBM compatible PC is an example of an Open system.

A Proprietary System is one in which all major components are from one manufacturer and the standards are often specific to that system and developed by the manufacturer. The early Macintosh models are an example of a Proprietary computer system as opposed to the IBM compatible.

Philosophy of Open Systems vs Proprietary Systems

An Open system allows a user to "mix and match" components and choose the most appropriate from a number of suppliers. The user is not beholden to any particular supplier and enjoys greater flexibility. A Proprietary system frees the user from the need to select components and allows the manufacturer to match the major components to give optimum performance. With a Proprietary system the responsibility for system performance rests with one supplier; with an Open system no one supplier is likely to take responsibility for overall system performance.

A key issue is standards. These standards will govern the interface between major system components. With telemetry and SCADA systems these standards govern the connections between; instrumentation and Remote Terminal Units (RTU) (this would cover connection type, contact ratings, isolation, current levels etc); RTU and communications bearer (covering impedance, communications protocol, signalling techniques and frequencies); and communications bearer to Master Station. The general rule is the simpler and more complete a standard is, the easier it is to adopt an Open system approach. A complex or incomplete standard will leave areas of ambiguity; major components may comply to the same standard but not connect well to one another because of the ambiguities and missing items in a standard.

In order to determine the suitability of an Open or Proprietary system approach a good understanding of the relevant standards is essential. What each standard covers is of particular importance. An example is RS-232. It is distressing to note the number of users, who upon noting that an RTU is equipped with an RS-232 port, expect that the RTU will be able to communicate with other serial devices. RS-232 covers electrical signal levels, signal flow control and is associated with various connector standards but it does NOT cover data format, speed of transmission, and the meaning of the data that is sent.

Comparison of Open and Proprietary Telemetry and SCADA Systems

We will use examples of each type as a basis for comparison. With complex systems there is no clear dividing line between Open and Proprietary systems so we will provide extreme examples of both types of system. Readers should be aware that there is a middle ground. Our example Open telemetry system consists of an RTU comprising a weatherproof case, power supply, battery, PLC, modem and radio. A repeater may feature in the communications path. At the base there is a radio, modem, IBM compatible PC and a SCADA software package. Case, power supply, battery, PLC, modem, radio, repeater, PC and SCADA software are all available from a number of suppliers. Our example Proprietary system consists of an "intelligent" high voltage circuit breaker / re-closer (with built-in RTU), full duplex 100% duty cycle radio link with high speed modem, specialised "intelligent" receiver and control panel at the base. These items are mostly manufactured by and all are supplied by the circuit breaker manufacturer.

The Proprietary system gives excellent high speed interaction with the circuit breaker at the substation. The Open system would just be able to give circuit breaker state and current. The Proprietary system has full control over the circuit breaker, including setting tripping profiles, and is fast enough for some complicated intertripping sequences. The Open system will have much more limited control function. Other manufacturer's circuit breakers will not be able to be monitored by the Proprietary system whereas the Open system will give the same information about each. The Open system will also be able to give other information about the substation such as transformer status, intruder access etc. An Open system can be expanded to cater for other applications in the substation.

It will take much longer to configure the Open system. This is true for all general purpose systems - they must be configured to suit each particular application. The Proprietary system is often application oriented and typically requires some fine tuning. If the application is too broad for the Proprietary system then it is very difficult to adapt it.

In the above examples there is a clear choice between flexibility and performance. In each case a minor concession could lead to a great degree of improvement. If a Proprietary substation oriented SCADA/Telemetry system is used then the user could have a much wider range of circuit breakers and other substation applications to choose from. Similarly, a more application specific RTU and SCADA package could enhance the performance of an Open system in an application for which the Proprietary system defined above is designed.

In many instances a major component of a system will have "special" features that are not covered by contemporary standards. This feature will therefore work with specific products (generally supplied by the same manufacturer). In the example above the ability to set a tripping profile is a "special" feature of the circuit breaker that is not typically supported by "off the shelf" RTU's and SCADA software packages.

Standards

An understanding of the standards governing these systems is important. The relevant standards cover the interconnection of major components. There is not room in this article to list and evaluate all the relevant standards but below is a description of the areas of concern and what should be covered. These can be broken into three main areas:

1. Interconnection between RTU and instrumentation.
Standards should cover voltages and currents, contact ratings, polarity, timing, connector types, wire diameters. There are a number of new instrumentation which have serial communication capability - the standards that cover this include connection (e.g. RS-232, RS-485, protocol (e. g. HART, Modbus) and format (e. g. the meaning and position of relevant data within each data message). With the exception of serial protocols most of the interfacing is well covered by standards and what is not is often easily modified to comply.

2. Interconnection between RTU and communications system.
The communications system can typically be radio, landline, Public Switched Telephone Network (PSTN), Digital Data Network, X.25, Optical Fibre. These communications bearers have vastly different characteristics and systems that work one may well not work on another. The standards should cover physical connections characteristics such as impedance, signalling strength, connector type and pinout. They should also cover data rate, frequency and size of data packets.

3. Interconnection between RTU and master system.
This area has the widest scope for standards. There are many areas that need to be covered and a good grasp of all the issues is required before "marrying" various RTU and base station components. The comms bearer is part of the interconnection and the relevant standards are outlined above. In addition there are the issues of communications protocol, and data format within the message transmitted via the protocol. A common mistake is to confuse the protocol with the format of messages. The communications protocol defines the data techniques to ensure that a message is successfully transmitted and received. The data format describes the meaning of the data within the transmitted message. An example: HDLC is a well defined and complex communications protocol which is designed to ensure reliable transmission of data in "information frames". HDLC does not define what is in the information frames - it is up to the user to put information in these frames. A data format specification would describe what is in the information frames so that the various devices such as RTU and base station computer can do something with it.

Problems and Pitfalls

With Open systems the major problem area lies with the integration of the major components of the system. This is often due to either the standards governing interconnection being incomplete or the components not fully complying with these standards. Also if the standards governing interconnection are too narrow then much of the functionality of the various major components may be lost. Also, if there is a major configuration effort or there is a major job required to connect the system's major components many of the advantages of flexibility are lost.

With Proprietary systems a major worry is becoming beholden to a single supplier/manufacturer. There are a number of aspects to this. The supplier/manufacturer may be slow to respond to technological change in a particular subsystem of the SCADA or telemetry system. An example of this was when the rapid growth of graphical user interfaces recently caught several manufacturers without the in-house ability to compete in this area. The customer is also at risk if the supplier / manufacturer goes out of business. The supplier /manufacturer is not under the same competitive pressure to keep prices down after the initial sale.

Conclusion

In general an Open system offers flexibility whereas a Proprietary system offers performance or security. The more complex a system is the more difficulties arise with standards. The factors governing the functionality and reliability of an Open system approach are the completeness and compliance with interconnection standards. A Proprietary system leaves the user vulnerable to a single supplier.