More answers now: Back to basics: What you need to know about today’s PLCs

PLC hardware tips, more answers, insights

• More programmable logic controller (PLC) advice follows the webcast, “Back to basics: What you need to know about today’s PLCs.”
• Additional audience questions about PLCs, not answered during the live webcast, answered webcast instructors with PLC expertise, are answered here.

More tips and advice about programmable logic controllers (PLCs) follows from an instructor for the Control Engineering webcast, “Back to basics: What you need to know about today’s PLCs.”

Learning objectives for the webcast were to:

• Assess how, where and why PLCs can benefit existing and new automation implementations.
• Examine types of PLCs available and how PLC designs vary in computing power, size, communications and other ways.
• Understand how PLCs benefit automation applications and connected devices and systems.

Webcast instructors were:

• Frank Lamb, founder and owner, Automation Consulting LLC
• Braden Hadwiger, mechanical engineer, Huffman Engineering Inc.

Webcast instructors answered audience questions during the end of the PLC programming webcast. Hadwiger and Lamb provided some additional answers below for questions that were not answered during the webcast.

Question: What should engineering programs teach students about PLCs?

Hadwiger: Engineering Programs should teach students the basics of what PLCs are and how they are used in industrial and utility settings.

Lamb: It is difficult to go far beyond the basics when students are taking multiple classes. An introduction to what a PLC is, the basics of IEC61131-3, the hardware layout and some hands-on practice in a few different languages are probably all students have time for.

Q: Can PLCs use EtherCAT?

Hadwiger: PLCs can use EtherCAT, but it must be supported by the PLC manufacturer.

Lamb: Yes, often for motion control.

Q: Can PLCs connect to SQL databases?

Hadwiger: PLCs can generally connect to SQL databases, however additional modules generally are needed to make this happen.

Lamb: Usually not directly unless they are industrial PCs. Some kind of SQL is probably necessary, which is not native to PLCs.

Q: How can I best plan around certain PLC manufacturers unexpectedly moving PLCs to end of life?

Hadwiger: The best way to plan around this is keep an eye on the product lifecycles from the manufacturer and to try and start planning for upgrades before a PLC is completely obsolete such as when it is designated as Active Mature to prevent being caught off guard.

Lamb: Pay attention to what the manufacturers are doing. They rarely “unexpectedly” move PLCs to end of life; either the technology is old and a new model has already come out, or the company goes out of business.

Q: Which communication protocol is the most used in factories?

Hadwiger: Generally a variety of ethernet based protocols are used. Serial communications are also commonly used.

Lamb: Some kind of Ethernet based protocol; Ethernet/IP, ProfiNet, Modbus TCP, EtherCAT are just a few. Some RS485 comms are still common such as Profibus, Modbus. IOLink, CCLink are also common.

Programmable logic controllers (PLCs) can be specified by their attributes, including those shown on this table, such as large rack, modular and brick, said Frank Lamb (left), founder and owner, Automation Consulting LLC, an instructor for the Control Engineering webcast, “Back to basics: What you need to know about today’s PLCs.” Courtesy: Control Engineering webcasts, Automation Consulting LLC.

Q: Do PLCs require UPS or a battery backup on their power supplies?

Hadwiger: PLCs do not require a UPS or battery backup, but this is highly recommended as it helps limit data loss when power is lost and code can be written in the PLC to help the system recover from the power loss. While you may not have motor power during a power loss, a UPS can keep the PLC powered up to ride through the power loss and allow the PLC to know it has lost line power whereas no UPS removes this ability.

Lamb: PLCs are able to retain their status and program without a separate UPS. They are already battery backed as far as the program goes. If you can’t run motors and activate actuators, the UPS won’t help.

Q: What are the typical equipment operating temperatures of PLCs and the auxillary supporting equipment within the enclosure?  Do we need to provide ventilation or air conditioning for the PLC control cabinets?

Hadwiger: Generally, PLCs are rated for -4 deg F to 32 deg F for minimum operating temps to 131 deg F for maximum operating temps, but this again varies by manufacturer. Environmental conditions should be taken into account when designing PLC control cabinets. Most enclosure manufacturers have calculators that can be used to determine if AC or ventilation is necessary.

Lamb: Temperatures without ventilation can easily reach 120F or higher within a cabinet, especially with power equipment like VFDs and motion control. A filtered fan is an inexpensive remedy. If the temperature is still high, an air conditioner may be needed.

Q: What are your experiences with particular automation vendors, and why are some more preferred in certain industries?

Lamb: Many perform similarly in many applications, such as motion control. Installed base and reputation go a long way for preference.

Q: What type PLC is most resistant to cyber-attacks?

Hadwiger: This depends mostly on the type of PLC. Some are built to meet certain cybersecurity standards. Generally, network security and user security are more effective in resisting cyber attacks than the PLC brand itself.

Lamb: Most PLCs have internal security configuration, but network security and authentication is most important to prevent access by unauthorized users.

Q: What is the significance of the term “edge” used to name Edge devices?

Lamb: The “Edge” is usually the outlying PLCs and devices doing the actual control. If a diagram of all interfaces and controllers is drawn, these are the devices at the “edge” of the diagram.

Q: What Ethernet protocols make a PLC ready for the future of industrial internet of things (IIoT)?

Lamb: Message queuing telemetry transport (MQTT) is coming on strong for IIoT applications. While not an Ethernet protocol itself, it is carried by other Ethernet protocols.

Q: Are there any PLC technologies on the way out the door? Or should be aware of ?

Lamb: Serial protocols like RS232 and RS485 are being replaced by Ethernet.

Q: What language is not recommended for PLCs anymore?

Lamb: Instruction List has been deprecated (not recommended) by the IEC. This includes STL.

Q: Is a particular I/O system dominating the market?

Lamb: IOLink is becoming the best way to aggregate IO outside of direct ethernet protocols, primarily because it is non-platform specific.

Q: Will Ethernet APL take over?

Lamb: It is strong in process control, though not so much for standard machine control. It lends itself to long cable lengths but is slower than other protocols.

Benefits of programmable logic controllers (PLCs), include ability to integrate with supervisory control and data acquisition systems (SCADA), flexible device integration and support for complex control loops, as explained by Braden Hadwiger, mechanical engineer, Huffman Engineering Inc., during the webcast, “Back to basics: What you need to know about today’s PLCs.” Courtesy: Control Engineering webcasts, Huffman Engineering Inc.

Q: What is your opinion on a fully virtualized PLC, regarding timeline of market adoption and use cases?

Hadwiger: There are a few options available for a full virtual PLC. The is more of a push in the marketplace for this, but I see adoption being slower due to control systems being separate from IT systems and a full virtual PLC running on a PC can create more integration of these systems which is not always desired. The clearest use case is code reuse as a fully virtual PLC could be agnostic to the physical hardware that is running the virtual PLC.

Q: What other popular trends you see in selection of PLCs for industrial projects?

Lamb: Higher speed, more memory are a result of technology advances. This can lower cost, but this is not always passed on by the manufacturer.

Q: How often do PLCs run out of program memory?

Hadwiger: Generally, PLCs do not run out of memory, but this can happen if a PLC is very undersized such as a PLC with 1MB of memory or less being used to run several process units.

Lamb: This is very rare any more with the advancements in technology. If the smallest controller was chosen for a large application, it can still happen. This would generally fall under bad design.

Q: Does NIST 800 allow for the implementation of PLC systems on Rasberry PIs or allow the use of “AI?”

Lamb: I believe there are only a few approved AI modules for critical networks. This may change in the future, but until AI is better understood, defined and controlled, it will not be widely permitted. They used to use the term “Fuzzy Logic” for some PID controllers, this scared some users off but was never disallowed. Raspberry PIs are just computers, and there are hardened versions available. If they run approved software they can generally be used.

Q: What can you say regarding PLC use for fast loops, such as reactors with runaway possibility. What is the scan time available?

Hadwiger: Scan times generally vary with the processors used and the size of the programs. Most modern large system processors can easily have less than 20ms scan times but again this is dependent on the processor and program size and composition.

Lamb: Typical scan times for modern PLCs are far lower than the time to move actuators. Periodic tasks can make scan times 5ms or less; outside of motion control (which is usually not calculated within the PLC), this is usually fast enough for physical movement. Safety should always be hardwired or in a safety PLC.

Q: Does ISA recognize PAC vs PLC term?

Lamb: They use the term in their documentation, but no one seems to have a clear definition of the differences.

Q: I like functional block programming language. Do you see any industry trends in selecting PLC programming languages?

Hadwiger: Generally, I see ladder logic being preferred if technicians or staff are getting online with a PLC to troubleshoot frequently as they are more familiar with electrical schematics. If this is not the case however, I see more of industry using function blocks as they are able to make the size of a routine smaller, they are easier to troubleshoot large sections of code, and are much easier to maintain from a code reuse/encapsulation perspective.

Lamb: Programmers will always have their favorites, but plant software specifications usually define what can or should be used for purchased custom equipment. OEMs can often use what they want.

Q: Can “bricks” be connected to expand the I/O capability of a design?

Hadwiger: This depends on the type of PLC being used but this could be done. Generally, additional racks or “brick” PLCs with extendable buses are used to expand the I/O capacity instead as the additional cards tend to be cheaper and easier to integrate than setting up communication between multiple PLCs.

Lamb: Yes, if a peer-to-peer communication protocol is used. It will probably be non-deterministic, so is not recommended for actual control.

Q: What about obsolescence relative to cybersecurity?

Lamb: Cybersecurity should primarily take place at the network level, so other than authentication/SSL I don’t see the necessity.

Q: Equipment obsolescence is a persistent issue. What is your experience migrating to PLC of the same brand; different brand? Migration/conversion software availability?

Hadwiger: Generally, manufacturers of a specific PLC will have some sort of migration path or additional product that can be used when a major product line is obsolete. Moving to a different brand tends to be a full program conversion. This can also be the case if the same manufacturer is used, but the product being replaced has been obsolete for a long period of time.

Lamb: Migrating to a different brand, at least for larger manufacturers, can be costly in terms of hardware, software and training. For this reason, I rarely see it done unless a principal gets tired of price-gouging.

Q: What open communication protocol is currently most widely supported? Which do you recommend and why?

Hadwiger: Generally speaking, Modbus is viewed as the most widely used “Open” communication protocol. It can be useful as it is very agnostic when it comes to communication between devices, but requires more setup by the user as the protocol just specifies where the data is located. This can be the best depending on your use case. Manufacturer specific communication protocols generally can be easier to use as more of the mapping work is done by add on files for the protocol, but these are not as open.

Lamb: Most communication protocols are only as open as the platform lets it be. While Modbus is probably the most widely used, it is difficult to set up and is still not supported on every platform, especially for Ethernet. My recommendation is dependent on the primary supported protocol by brand; EtherNet/IP, Profinet and EtherCAT. I also like IOLink a lot, and you can get there on all of the previous protocols.

Edited by Mark T. Hoske, editor-in-chief, Control Engineering, WTWH Media, mhoske@wtwhmedia.com and webcast moderator.

KEYWORDS

Programmable logic controllers, PLCs, PLC webcast

LEARNING OBJECTIVES

• Understand that the following webcast is online for a year after the Nov. 6 broadcast of “Back to basics: What you need to know about today’s PLCs.”
• Explore additional answers from one of the webcast instructors beyond the question-and-answer session during live webcast.

CONSIDER THIS

Do you know all you need to know for efficient PLC integration and operation?

ONLINE

Miss this PLC webcast? It’s archived for a year after the Nov. 10 broadcast date:

https://www.controleng.com/webcasts/back-to-basics-what-you-need-to-know-about-todays-plcs

Control Engineering provides more on PLC topics.

https://www.controleng.com/control-systems/plcs-pacs

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