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Case Study: RS-485 Robust Industrial Control Communications System

Introduction

RS-485 Overview

RS-485 is a low cost, standardized solution for industrial Local Area Network
(LAN) communcations.
RS-485 supports full duplex operation upto 10 Mbit/s, over short distances.
The signaling technology allows for a trade-off between bit rate and distance.
Distances of several hundred meters can be traversed in a single hop but at a
correspondingly lower data rate.
The project brief was to develop an industrial communications system for a site wide Supervisory Control and Data Acquisition (SCADA) system, with dual redundant robust communications.

The system included four inter-connected motor control drives, each controlling a single diesel powered D.C. generator, and a central control station.
All of the nodes in the network share data via the network using the industrial RS-485 standard, see side-bar.
For reliability and redundancy, the communication system was built around a bi-directional ring architecture, with every single communications path taking a different physical route between the power stations.
Each individual link in each ring is a direct point-to-point link between the transmitting node and the next receiver.
This is shown in the following diagram:

RS-485 Industrial Network Topology
Figure 1: Node Diagram

The data frames always flow round the rings in the directions shown and at each stage frames are read and written to the ring.
The following diagram shows the structure of each data frame:

RS-485 Industrial Network Data Frane Structure

Figure 2: Data Frame Structure

Each node in the network receives data frames from each ring are read into the data receive thread, where the checksum is verified.
The data in the sub-frames for each controller are timestamped. The timestamps for the sub-frames received on each ring are compared and the most recent is selected.
This data for each remote controller is written to the combined data frame in local memory.
The example diagram shows the scenario for controller #3, whereby the remote data for controllers #1, #2 and #4 are received and written to the combined dataframe and the data for the local controller (#3) is written by the local sensor manager thread.

RS-485 Industrial Network Local Node Dataflow
Figure 3: Local Node Dataflow

The above architecture was chosen because it simplified the overall architecture and meant that the data transmit thread was identical on all controllers, which made software development, documentation and maintenance a lot simpler.

The physical RS-485 network interface was handled via an optrically isolated DIN rail mounted modue, for safety and ease of installation reasons respectively.
While multi-channel RS-485 interfaces are available, in this application, single channel devices with independent 12V DC power supplies, for redundancy purposes.

RS-485 Industrial Module
Figure 4: RS-485 Industrial Module Similar To The One Used In The Application

Summary

The longest data run in the network was 210 meters and the network was able to run at 1M bps over CAT-5 cable.
RS-485 proved to be very robust in this harsh environment and the dual-redundant network lead to even higher reliability.

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