Supervisory Control and Data Acquisition (SCADA) refers to the process of gathering information, often in real time, from remote locations. The data comes from oil wells, pipelines, electric power grids, traffic signals, seismic sensors, air quality monitors, etc. This information is used to analyze and control the performance of these systems. For example, oil well information might include the volume of liquid pumped over a period of time, temperature, pressure, contaminant levels and the amount of power used. The control aspect may include controls to a well pump to increase or decrease output, or shut down altogether. SCADA systems and the network environment they operate in are changing. Typically small amounts of data are recovered from many remote points at low speed, usually 1200 bps. The latest SCADA equipment has more computer power and complexity. More complex data and control of many functions requires higher speed, so systems are upgrading to 9600 bps or even higher speeds. SCADA systems are typically designed for long life, 10 years or more, and all components have to be designed for the harsh outdoor environments they are used in. The major components of any SCADA system begin with the host computer and the application software, which controls the Remote Terminal Unit’s (RTU’s) or Meters. The software will need a means to connect to each remote device one at a time. This is usually done with a standard protocol fitting the application. The most complex part of the system is the communications media, connecting the host to all the remote devices. This is composed of the landlines, cellular links and the modems needed to interface the data to the analog lines. The communications media frequently represents a monthly expense if leased from a public utility.
SCADA applications require the most robust protocols and
control systems in existence considering that they are used in very harsh
environments and must operate flawlessly 24 by 7 for years at a time. Most
SCADA systems use polling to address multiple devices on a single network.
Polling is efficient since many devices can be attached to a common network.
The National Transportation Communications for ITS Protocol (NTCIP) family of standards defines protocols and profiles that are open, consensus-based data communications standards. When used for the remote control of traffic signals, roadside and other transportation management devices, the NTCIP-based devices and software can help achieve interoperability and interchangeability among a number of manufacturers. Many SCADA systems rely on a defacto standard, Distributed Network Protocol (DNP). Modicon’s MODBUS is also very popular and has a large supporting user group. There are many other protocols usually proprietary for control of specific remote devices. The Electric, Gas, and Water Utilities have standardized on Itron’s MV-90 protocol for automated meter reading and control of remote meters, recorders and other devices. Some newer data networks are based on Ethernet and IP protocols, frequently acting as transport for the older protocols, requiring new physical interfaces and software for existing SCADA devices. There are still many issues regarding the Internet, and it’s reliability when used for SCADA applications.
Because of its need for reliability, most SCADA systems use telephone company leased lines, private wire, and private optical cable networks. More recently new types of networks, including Cellular and sometimes private Microwave are used. Cellular phone technology is very useful for connecting very remote locations, or difficult to access equipment. Leased lines are available in almost all locations, and can be configured as point to point or multidrop. Special modems are available to connect the components of SCADA systems to the network. Point to point modems offer analog speeds to 56 KBps, and device speeds up to 230.4 KBps. The higher data speed is accomplished using sophisticated data compression techniques. Multipoint or Multi-drop networks require polled modems. In the past the well known Bell 202 or V.23 modems were used with a nominal speed of 1200 or 1800 Bps. There are only a few multipoint modems in the world that are faster. One example is Telenetics DSP9612. This polled modem will emulate V.23 at 1200 / 1800 Bps as well as 9600 Bps making it suitable for older networks, or upgraded faster networks. This modem can acquire carrier from a polling modem in 23 ms or less, so little time is lost in the network, and a larger number of points can be polled.
Add illustration here.
The most important modem characteristics in SCADA systems are the operational temperature range, resistance to vibration, flexible power supply options, and intrinsic reliability. Telenetics manufactures modems specifically for the SCADA environment in both point to point and multidrop configurations with versions for use with either AC or DC power, as may be available at remote locations. The power sourse at remote sites needs careful consideration. If the application requires 99.9% availability, standard AC power may not be suitable. The power companies will usually state 99% availability maximum. A 1% power off rate amounts to more than 87 hours of off time per year. Batteries and an AC charging system are usually required for high availability systems. Batteries also have peculiarities, such as failure to operate in cold climates. As the temperature decreases, the batteries ability to supply current declines drastically.
SCADA networks require careful consideration of each component, the network connecting the components, and the available power. Cost must be considered when selecting the communications medium. The public Telephone companies can be expected to provide favorable rates for SCADA systems, which represent a major business segment in their territories. If wire is not available there is the option of cellular communications. In some cases, such as power sub stations, cellular has the advantage of an indirect connection, by-passing the requirement for expensive de-coupling and protection for physical phone lines, which is very expensive. The modems and the power source must be rugged and the specifications for this equipment must exceed the temperature extremes anticipated at the installation.