Industrial Data Communications

The challenge for the engineer and technician today is to make effective use of modern instrumentation and control systems and ‘smart’ instruments. This is achieved by linking equipment such as PCs, programmable logic controllers (PLCs), SCADA and distributed control systems, and simple instruments together with data communications systems that are correctly designed and implemented. In other words: to fully utilize available technology.
Practical Data Communications for Instrumentation and Control is a comprehensive blog covering industrial data communications including RS-232, RS-422, RS-485, industrial protocols, industrial networks, and communication requirements for ‘smart’ instrumentation.
Once you read this post, you will be able to analyze, specify, and debug data
communications systems in the instrumentation and control environment, with much of the material presented being derived from many years of experience of the authors. It is especially suited to those who work in an industrial environment and who have little previous experience in data communications and networking.
Typical people who will find this blog useful include:
• Instrumentation and control engineers and technicians
• Process control engineers and technicians
• Electrical engineers
• Consulting engineers
• Process development engineers
• Design engineers
• Control systems sales engineers
• Maintenance supervisors
I would hope that you will gain the following from this blog:
• The fundamentals of industrial data communications
• How to troubleshoot RS-232 and RS-485 links
• How to install communications cables
• The essentials of industrial Ethernet and local area networks
• How to troubleshoot industrial protocols such as Modbus
• The essentials of Fieldbus and DeviceNet standards
You should have a modicum of electrical knowledge and some exposure to industrial automation systems to derive maximum benefit from this blog.
Why do we use RS-232, RS-422, RS-485 ?
One is often criticized for using these terms of reference, since in reality they are obsolete. However, if we briefly examine the history of the organization that defined these standards, it is not difficult to see why they are still in use today, and will probably continue as such.
The common serial interface RS-232 was defined by the Electronics Industry Association (EIA) of America. ‘RS’ stands for Recommended Standards, and the number (suffix -232) refers to the interface specification of the physical device. The EIA has since established many standards and amassed a library of white papers on various implementations of them. So to keep track of Preface xii them all it made sense to change the prefix to EIA. (You might find it interesting to know that
most of the white papers are NOT free).
The Telecommunications Industry Association (TIA) was formed in 1988, by merging the telecom arms of the EIA and the United States Telecommunications Suppliers Association. The prefix changed again to EIA/TIA-232, (along with all the other serial implementations of course).
So now we have TIA-232, TIA-485 etc. We should also point out that the TIA is a member of the Electronics Industries Alliance (EIA). The alliance is made up of several trade organizations (including the CEA, ECA, GEIA...) that represent the interests of manufacturers of electronics-related products. When someone refers to ‘EIA’ they are talking about the Alliance, not the Association!
If we still use the terms EIA-232, EIA-422 etc, then they are just as equally obsolete as the ‘RS’ equivalents. However, when they are referred to as TIA standards some people might give you a quizzical look and ask you to explain yourself... So to cut a long story short, one says ‘RS-xxx’and the penny drops.
In the blog you are about to read, the authors have painstakingly altered all references for serial interfaces to ‘RS-xxx’, after being told to change them BACK from ‘EIA-xxx’! So from now on, we will continue to use the former terminology. This is a sensible idea, and we trust we are all in agreement!
Why do we use DB-25, DB-9, DB-xx ?
Originally developed by Cannon for military use, the D-sub(miniature) connectors are so-called because the shape of the housing’s mating face is like a ‘D’. The connectors have 9-, 15-, 25-, 37- and 50-pin configurations, designated DE-9, DA-15, DB-25, DC-37 and DD-50, respectively. Probably the most common connector in the early days was the 25-pin configuration (which has been around for about 40 years), because it permitted use of all available wiring options for the RS-232 interface.
It was expected that RS-232 might be used for synchronous data communications, requiring a timing signal, and thus the extra pin-outs. However this is rarely used in practice, so the smaller 9-position connectors have taken its place as the dominant configuration (for asynchronous serial communications).
Also available in the standard D-sub configurations are a series of high density options with 15-, 26-,44-, and 62-pin positions. (Possibly there are more, and are usually variations on the original A,B,C,D,or E connector sizes). It is common practice for electronics manufacturers to denote all D-sub connectors with the DB- prefix... particularly for producers of components or board-level products and
cables. This has spawned generations of electronics enthusiasts and corporations alike, who refer to the humble D-sub or ‘D Connector’ in this fashion. It is for this reason alone that we continue the trend for the benefit of the majority who are so familiar with the ‘DB’ terminology.