HPR4758: Programmable Logic Controls - Episode 6

01 Introduction This is the sixth episode in an 8 part series. 02 In the previous episode we looked at I/O modules, what they are, how they work, and the purpose that they serve in the system. We also discussed machine safety systems and also claims made by certain people about cyber security and industrial safety. In this episode we will look at PLC networking, together with a detailed look at one network protocol, Modbus. 03 Types of Networks PLC networks fall into three broad categories, based on the purpose of the network. These categories are • PLC to IO • PLC to PLC • and PLC to PC or Personal computer. There is a great deal of overlap between these, so don't expect them to fall into nice discrete pigeonholes. 04 A Multiplicity of Protocols Logically, you would think that the advantages of interoperability would be so great that there would be just a few networking protocols and everyone would use them. The reality is the opposite. There is such a multitude of protocols that I won't attempt to list them all. Industrial automation is a market that is firmly in the control of the vendors, not the customers, and the vendors value customer lock in over user benefits. 05 What I will focus on instead is the technological development over time so you can understand how we ended up where we are today. I will illustrate this with a few protocols from a few vendors before going into detail on one specific family of networks. 06 Basic Point to Point Serial The first and most basic networks were simple point to point serial links. You may argue that a point to point link is not a network, but in this case it is the start of networking with PLC, and one of the most important industrial network protocols today is a direct descendent of one of the early point to point protocols. 07 These point to point links initially addressed two main issues. One was that once computers were being used to run the software needed to program PLCs, some means of downloading the program to the PLC and to debug the PLC program was needed, and this took place over a point to point serial link. 08 The other was that there were various microprocessor driven instruments and devices where it was desirable to allow the PLC to communicate with them in order to obtain readings, adjust set points, and otherwise exchange data. A serial link was the obvious means of implementing this. The physical and electrical media were generally RS-232, RS-422, or serial current loop. 09 RS-232 RS-232 should be familiar to most listeners. The notable characteristic of RS-232 is that the data and control signals were referenced to ground, or "single ended" as this is also known. While this was simple, it placed limitations on length of cabling and it was somewhat problematic when the PLC and the other device were connected to different power sources and did not have a common ground. 10 RS-422 RS-422 differs from RS-232 in that the signals are differential rather than referenced to a common ground. Signal lines come as pairs, and it is the difference in voltage within the pair rather than compared to ground which defines the zeros and ones. This allows for higher data rates, up to 10 mbps at very short distances, and much longer cable runs, up to 1,200 metres, at the cost of lower data rates. 11