Technical Paper
RS-485 Serial Data Communication - Physical Layer for Factory Automation and Process Control Networks
Mark E. Hazen
High Performance Analog
Intersil Corp.
Today, there are some predominant and widely deployed factory automation and process control networks, such as PROFIBUS and Field Bus, which employ RS-485 (EIA-485) serial data communications transceivers, and the 485 standard, in the physical layer. These transceivers enable control of manufacturing and other processes over a wide area, such as the factory floor (field), from a central computer control station. The simple twisted-copper-pair network wire (bus) snakes its way around the plant (field) throughout the machinery to a host of sensors and actuators that the computer uses to direct and syncopate a myriad of operations that yield a final product. Already you can see the derivation behind the popular network name, Field Bus.
The elegance of such a network is found in its low cost, simplicity, extensibility and very high noise immunity, in addition to the tremendous selection of sensors and actuators readily available to bring the network online quickly. PROFIBUS is an industry name coined from Process Field Bus. The PROFIBUS association is made up of over 1,200 members internationally and is responsible for thousands of software and hardware network automation products.
The Electronics Industry Association (EIA) Recommended Standard (RS) number 485 is the foundational hardware fabric for Field Bus networks. It is designed to provide bi-directional, half-duplex, multi-point data communications over a single two-wire bus. Full-duplex operation is accomplished using a four-wire, two-bus network that completes the process loop from sensor to processor to actuator, and so on. The data bus can be up to 4,000 ft in length with a data rate at approximately 100 kbps. The maximum data rate is 10 Mbps, or so, for short runs, trading off distance for speed.
Figure 1. Differential signaling in a multi-drop RS-485-based network.
As shown in Figure 1, RS-485 drivers and receivers utilize differential data communications, also referred to as balanced differential signaling. The driver uses two wires over which the signal is transmitted. However, each wire is driven and floating separate from ground, meaning, neither is grounded as in a single-ended system. Correspondingly, the receiver has two inputs, each floating above ground and electrically balanced with the other when no data is being transmitted.
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