Papyrus Computer Technologies Ltd.                             

What is an industrial bus? Traditionally, the industrial bus has been used to allow a central computer to communicate with a field device. The central computer was a mainframe or a mini (PDP11) and the field device could be a discreet device such as a flow meter, or temperature transmitter or a complex device such as a CNC cell or robot. As the cost of computing power came down, the industrial bus allowed computers to communicate with each other to coordinate industrial production.

As with human languages, many ways were devised to allow the computers and devices to communicate and, as with their human counterpart, most of the communication is incompatible with any of the other systems. The incompatibility can be broken into two categories: the physical layer and the protocol layer.

The physical layer and the protocol layer can be defined using the phone system as an example. Any spoken language can be carried over a phone line. As long as both the speaker and the listener(s) understand the language, communication is possible. The phone line is not concerned with the meaning of the signal that it carries, it is merely moving those signals from one point to another physically. This is the physical layer, the conduit in which communications pass from one point to another. On the other hand, the speaker and listener(s) are concerned with what is transported over the phone line. If the speaker is talking in Spanish and the listener(s) are only fluent in English, communication is not possible. Although the physical layer is working, the language or "protocol" is not correct, and communications cannot exist. The industrial world has developed a variety of different physical and protocol communications standards. A list of all of them would fill the rest of this article, so we will limit this discussion to industrial buses using the RS-232 and RS- 422/485 standards for their physical layer.

The greatest difference between RS-232 and RS-422/485 is the way information is transmitted. (See Technical Article: Getting from Here to There: Serial Communications Systems for more information) RS-232 uses a single-ended, bipolar voltage to move data between two points. RS-422/485 uses a balance differential pair to accomplish this same task. The advantage of using RS-422/485 in an industrial environment is greater noise immunity. This allows a greater distance between the transmitter and receiver. There is a downside to the greater distances provided by RS-422/485- the "difference of potential" between end points.

Industrial buses cover a large area. Often different areas of the network are supplied by different power sources. Even though all of the sources are grounded, a voltage difference can exist between the grounds of these voltage sources. This voltage difference can upset the data line in an RS-422/485 bus by pushing the signal voltage out of range and, in some cases, an excess voltage can damage equipment. Another source of excess voltage potential can be caused by intermittent sources. Power line surges and lightning are causes of this type of disturbance, but other causes, such as large electric motors starting and stopping, can temporarily affect the ground reference voltage. The solution to this problem is to employ RS-422/485 devices that provide isolation between different parts of the network. (See Technical Article: Data Line Isolation Theory for more information) Additional protection can be achieved by using a fiber optic link between the network and areas known for voltage problems such as a power house or a water treatment plant.