Distributed Control Systems, PLCs, and Networking

Whether it be with a customer, a hiring manager, or even your pet, communication is key, and the automation realm is no different. Since the dawn of the computer age, the goal of all IT systems is to relay information from one electrical device to another. The early 2000s saw this concept explode with the widespread adoption of the internet. With the cost of computing drastically decreasing and automation controllers becoming significantly more powerful, point-to-point communication within an automation system has become paramount.

With the way most manufacturing environments now operate, it is not uncommon for many different types of automated controllers to be networked together for coordination. Even isolated systems still use networking technologies to communicate with different parts of the machine such as motor drives, power supplies, and so on. If you want to be an automation engineer, or at least grow as an automation developer...

Technical requirements

Unlike the other chapters in this book, this chapter will be only theoretical in nature. We will explore concepts but not develop code. There is no code that you need to worry about pulling down.

What are computer networks?

If you’re reading this book, chances are you know what a computer network is. This is because to function in the 21st century, you have to be able to use the internet, and the internet is nothing more than a global computer network. So, with that being said, what is a computer network? In short, a network is a way for computers and other electrical devices (such as printers, smartphones, tablets, or any other devices with either wired or wireless capabilities) to be able to communicate and share data. In other words, a computer network can best be thought of as a bunch of devices that are wired together so they can talk to one another.

Network topology

To get into a more advanced networking concept, we need to explore topology. In short, a network topology is a way a network is built. So, you can think of a topology as a layout of all the devices on a network and the way they are interconnected. In other words, a topology is a network blueprint...

Common IT protocols

Electrical devices inherently do not understand any form of language that we humans can speak. All a device such as a computer or PLC understands is whether a particular pin is energized at a given time or not. Specific languages known as protocols have to be used so that one device knows what the other device is saying. There are many different protocols out there. Some are very common and are used everywhere, while some are proprietary and are only used with specific devices. The following section is going to be dedicated to exploring two common, everyday protocols that can be used with PLCs and other automation devices. These protocols can be used directly or as an underlying system for more specific protocols. As such, the first protocol stack that we will explore is TCP/IP.

TCP/IP

Arguably the most common form of computer communication is called the Transmission Control Protocol (TCP). TCP is one of, if not the most commonly used communication protocols...

PLC/automation device communication

UDP and TCP are general communication protocols. By this, I mean that they are used for many different types of IT applications, such as internet applications, common computer networks, and so on. However, many of the PLC manufacturers produce their own communication systems to be used with their PLCs and various types of industrial components. Some of these systems are very similar and use the same physical connectors as standard computers do – for example, Ethernet cables. However, some use exotic connectors and will be unique for certain devices. The first communication protocol we are going to discuss is one of the most popular, which is called Modbus.

Modbus

Modbus is an industrial communication protocol. Modbus is a little different than the other protocols that we have discussed thus far. Where TCP and UDP are more agnostic in terms of IT applications, Modbus was developed in the late 70s for use in PLC communications by the company...

Understanding distributed control systems

For many industrial processes, individual processes can vary greatly across a geographical location. For example, consider a bottled water bottling center. Let’s assume the treatment process involves the following steps:

1. Run a pump to collect water from the local lake.
2. Open the water intake valve and intake the water into a heating tank to boil the water.
3. Take the boiled water and add minerals to the treated water.
4. Bottle the water.

For a process like this, which has four intermittent steps, multiple PLCs would need to be used. However, there is a catch to this process. The four steps are going to take place over the bottling plant, which by definition is geographically dispersed. Since the bottling process can be thought of as a single process, we’re going to need a way to control the whole process. Enter the world of DCSs.

A DCS is very similar to a SCADA system. A DCS is essentially a coordinating...

The differences between DCSs and PLCs

The line between PLCs and DCSs is beginning to blur. However, DCSs and PLCs are two separate types of controllers. Due to the differences in the controllers, the overall applications for the controllers are different. To start the exploration between the two types of controllers, it is important to remember that a DCS controller is designed to supervise many PLCs, and as such, they are used to oversee entire processes.

Due to the supervisory nature of DCSs, they are usually not suited for controlling an individual process; they are much too slow. In terms of individual processes, a PLC is much more responsive and capable to make close to real-time adjustments. A DCS, on the other hand, is meant to (and usually does) supervise multiple systems, so it usually cannot handle the quick response time necessary to oversee a singular process.

Though response time is important, a DCS is much more scalable. In short, the number of I/O ports that a...

Summary

In conclusion, we have covered communication protocols, network topologies, DCSs, PLCs, and when to use each one. Networking is the backbone of many distributed systems. If you opt to use a DCS, networking will be used for many different distributed tasks. DCSs are also widely used in automation systems where distance matters. In all, to be an automation engineer, you must understand these concepts.

You have reached the end of this book; congratulations on that! This book was a crash course on the more advanced concepts that you will encounter in an advanced automation programming/engineering job. This book mostly focused on the software and IT side in general. With the current trend in automation, these are the concepts that you will want to focus on.

By this point, you should be exposed to many of the more advanced concepts of automation programming. Most of the concepts that were explored in this book are traditional software engineering concepts. Due to the nature...

Questions

Answer the following questions based on what you've learned in this chapter. Cross-check your answers with those provided at the end of the book, under Assessments.

1. What is a DCS?
2. When should you use a PLC over a DCS?
3. What data transmission rates does DeviceNet transmit data at?
4. What is the difference between Profinet and Profibus?
5. What is the default topology for Profinet?
6. What is the difference between TCP and UDP?
7. Which is faster: TCP/IP or UDP?
8. Which communication protocols can use standard Ethernet cables?
9. What are the resistor sizes that are required at the end of a DeviceNet trunk?

Further reading

Have a look at the following resources to further your knowledge:

• What is network topology? Best guide to Type & Diagrams: https://www.dnsstuff.com/what-is-network-topology
• What is TCP?: https://www.fortinet.com/resources/cyberglossary/tcp-ip
• The User Datagram Protocol (UDP): https://erg.abdn.ac.uk/users/gorry/course/inet-pages/udp.html
• Mary Dixon, What is DeviceNet?: https://realpars.com/devicenet/
• DeviceNet – designed for factory automation: https://www.can-cia.org/can-knowledge/hlp/devicenet/
• Mondi Anderson, What is EtherCAT?: https://realpars.com/ethercat/
• Michael Bowne, The difference between Profibus and Profinet: https://us.profinet.com/the-difference-between-profibus-and-profinet/
• Modbus ASCII vs Modbus RTU vs Modbus TCP/IP: https://theautomization.com/modbus-ascii-vs-modbus-rtu-vs-modbus-tcpip/
• Distributed control systems (DCS): https://www.techtarget.com/whatis/definition/distributed-control-system
• What...