More answers on PLC hardware speed, I/O, communications, redundancy

In a November 9 webcast on applications for programmable logic controllers (PLCs), Black and Veatch’s David Ubert and PLCopen’s Eelco van der Wal discussed best practices related to PLC hardware selection.

By Control Engineering November 21, 2023


Learning Objectives

  • Understand where (types of applications) PLCs can be effective.
  • Review PLC features and learn best practices related to PLC hardware selection.
  • See examples of how PLC hardware upgrades and selection helped applications and why.

Programmable logic controllers (PLCs) continue to be a popular choice for control systems in various industries due to their reliability, flexibility and other specific features that make them well-suited for certain applications. Beneficial features of modern PLCs include their modularity and scalability, which allows for easy expansion and modification, allowing users to add or remove components as needed without making burdensome changes to the overall system; their capacity for customizable logic, which allows control to be modeled around the requirements of specific processes or applications; and their ability to integrate with existing systems. Due to their ability to interface with a variety of sensors, actuators and other devices PLCs are often chosen for retrofit projects or for integrating with existing control systems.

Moreover, while PLCs are more common in discrete manufacturing, distributed control systems (DCS) is often used in process industries such as oil and gas, petrochemical and power generation. However, PLCs are still employed in certain process control applications within these industries, especially when a mix of discrete and process control is required.

In a November 9, Control Engineering webcast, “PLC series: PLC hardware speed, I/O, communications, redundancy,” David Ubert, senior automation specialist at Black and Veatch, and Eelco van der Wal, managing director at PLCopen, discussed the types of applications where PLCs can be effective, reviewed PLC features and best practices related to PLC hardware selection, and provided examples of how PLC hardware upgrades and selection have helped specific applications. Questions submitted by the audience that were not answered during the live event are addressed below.

When will PLCs be fully conversant with BACnet?

David Ubert: PLCs can communicate over BACnet protocol by use of third party gateways. However, there are not typical protocols for PLC manufacturers to incorporate into their products. You would need to reach out to each specific manufacturer to get this information.

In regard to communication systems, what is recommended, serial communication or non-serial communication?

David Ubert: Each design is unique and would need to be investigated independently. However, Industrial Ethernet has been an increasing trend in the PLC communication arena.

What are the different types of communication protocols?

Eelco van der Wal: There are many. Nowadays, the most important are TCP/IP, UDP and HTTP

David Ubert: There are many different protocols used by PLCs to communicate. Some are used in peer-to-peer communication, while others utilize a master-slave protocol. Each manufacturer supports a different set of protocols, even within their own brand. Consult the specific PLC manufacturer for a list of supported protocols.

Is the processing speed of a PLC solely based on its clock speed?

Eelco van der Wal: Not always. It can also be related to the cycle time of the network. In the case of a network with remote I/O, it makes sense to synchronize both cycle times.

David Ubert: The PLC has a processor used to run the compiled program. In addition, there are other systems that can affect the overall processing speed such as I/O communication (local and remote).

Can a PLC be used in all applications?

Eelco van der Wal: At least in many. However, I do not see them move into telecommunications systems, or replacing machine learning (ML) and artificial intelligence (AI) types of applications. Similarly, I would not imagine them having applications in financial systems.

David Ubert: PLCs can be used in most any control system application that processes I/O. Limitations of use would be those that are cost prohibitive.

Are PLCs limited in their use due to limited communications capabilities?

Eelco van der Wal: Actually, there is not much of a limit in the communication capabilities with network speeds at 1GB/sec and support for multiple communication lines. However, they are not like Ethernet switches, which are dedicated to this type of communication.

David Ubert: They can be. However, third party resources can be used to overcome communication limitations.

Is it true that a redundancy can only be achieved by using multiple PLCs in a system?

Eelco van der Wal: Actually, yes. This can be up to multiple complete and independent systems, even if they are running different operating systems. However, normally they are the same, but separated. Be aware that 1oo3 systems do not have to be better than 1oo2 systems, as the first comprises three sets of systems which can generate more errors due to the higher hardware count.

David Ubert: When you configure multiple PLCs in a control system as a redundant pair, you will have processor redundancy only. Redundancy may still be desired regarding power supplies or I/O as well.

Are there many applications where PLCs cannot perform well due to processor limitations?

Eelco van der Wal: Not many. It is dependent on the supplier, as I see 128 core PLC processors that outperform your office PC by far. However, there are not many suppliers of these.

David Ubert: When PLCs first came out, they had limitations, but today’s manufacturers use electronics that can perform very well for most applications.

Is MODBUS commonly used for a communications protocol? What about HTTP?

Eelco van der Wal: Yes, it is. However, they are now replaced by newer communication structures like Asi and IO-Link on the lower level and OPC UA at the top level.

David Ubert: Historically, Modbus has been a very popular protocol for PLC communication, and it still is. It is a very simplistic protocol and does not have good cybersecurity protection. Newer protocols such as industrial Ethernet DNP3 are quickly replacing this protocol. HTTP is not a protocol used in PLC control applications.

Please comment on applications and examples involving safety PLCs and criteria for their use, inclusion in design, and main requirements of design that must be addressed within a safety context.

Eelco van der Wal: There are different levels of safety. For instance, for a machine it can be limited to an emergency stop, and combined in a production line there can be safety curtains and procedures to enter the non-safe area. For large process control situations where the number of casualties can be much higher, a much higher level is applicable. Check IEC 61508, for instance.

Can we add different communication module blocks (like ethernet, BACnet, etc.) in the same PLC?

Eelco van der Wal: Yes, some suppliers support this, especially with a difference between the sensor/actuator level and the higher levels.

David Ubert: Most if not all the major manufacturers and high-end PLCs support multiple protocols. Gateways can also be used to support multiple protocols on PLC control networks.

What factors determine the appropriate type of I/O module for a PLC system?

David Ubert: There are several factors. When space is limited, high density I/O should be employed. It is good practice to try and standardize on I/O types so that the amount of spares required to support the system are few. The type of power available (120 VAC or 24 VDC) influences I/O selection.

Will IPC’s take over PLC’s in the near future?

Eelco van der Wal: No, there are too many different application with their own requirements. I assume that you refer to IPCs as an industrial PC with a windows environment. These applications are limited. RT-Unix based systems are more accepted.

David Ubert: I would not expect that to happen in the near future, or in the distant future.