The Simple Way to Retrofit Your Programmable Logic Controllers (PLCs) with IoT

Common Wireless PLC Uses 

While the usefulness of IoT-enabled PLCs is an endless field of possibilities, there are common patterns we see in the market such as remote monitoring stations, equipment monitoring, and commercial control systems. 

Remote monitoring stations  

PLCs are often field deployed, away from connectivity. These PLCs may reside in a pumping station, a storage facility, or in a tank designed to support a remote facility. In all three cases, connecting the PLC to the internet would improve the awareness of remote conditions and allow for remote control. Take, for example, a flood control system based on pumps and actuated sluices. These systems are located where they can provide the most flood control, which isn’t necessarily in the most convenient location for connectivity. Using the cellular network means you get wireless connectivity without having to build out your own connectivity infrastructure. 

Equipment monitoring

PLCs can also oversee turning on/off equipment based on conditions. It is important to be able to monitor the status of equipment remotely to ensure proper functionality. By wirelessly enabling a PLC, you can send the status of controlled machinery or equipment directly to your cloud, or even to your cellphone through SMS messaging. If, for example, a generator is supposed to turn on under certain conditions, it would be a good idea to monitor that system to ensure it’s in the proper state based on the conditions. That information could then be sent to a cloud dashboard or application where it can be reported on and audited. 

Facility control 

In a commercial context, it is important to control key functionality remotely. In the case of a facility that operates on a variable schedule, there could be economic efficiencies gained by disabling systems when they aren’t needed. By connecting commercial HVAC PLCs wirelessly, facility management can disable or control the HVAC system without a site visit, allowing for just-in-time adjustment.  

Considerations When Adding Wireless Capability to PLCs 

Digital transformation enables businesses to improve operational efficiency, but this requires connecting devices to modern systems. There are many considerations when it comes to upgrading a system, but PLC users have 3 choices when it comes to digitization: 

1. Do nothing. Leave PLCs unconnected. This is the cheapest strategy in the short term but doesn’t advance digital strategy. Any adjustments to PLCs must be made in the field. Data available at the PLC level is trapped at the PLC level and can’t easily be made available to information systems. 

1. Buy new PLCs with wireless capabilities. Some feel like this is their only option, but the risk and expense involved in completely replacing existing PLCs is intimidating, and often unnecessary.  

1. Retrofit PLCs for IoT connectivity. This strategy allows working PLCs to stay in place and presents less risk, less rework. The cost is less than replacing your PLCs with new ones but varies as this can be done with an off-the-shelf system or by building your own. 

When adding wireless capability, it is necessary to create a bi-directional flow of data from the PLC to a cloud application (or database) and back to the PLC. Each communication pathway serves a distinct but essential function: 

PLC to Cloud 

When a PLC is connected to a cloud application wirelessly, you can query the PLC for tags, then send the tag information to the cloud. Common uses here would be to monitor a system for conditions like storage tank level, anomaly detection, power supply availability, and triggered alarms. 

Cloud to PLC 

When a cloud application or database is connected to a PLC, you can avoid site visits by controlling the PLC remotely, changing set points, altering variables, and controlling state. Common uses here would be to reconfigure PLCs remotely, or even trigger functionality like opening or closing a valve, actuating a solenoid, or turning on/off an electric system. 

Once connectivity is established, you can interact with any parameters that are exposed to the PLC Modbus communication interfaces. There are many modems and modules on the market that can connect your PLC to the IIoT.  

Retrofitting Legacy PLCs: Build vs Buy 

There are off the shelf products available for retrofitting a legacy PLC with wireless connectivity, but you may find that you want to augment the PLC behavior or custom build a device.  

Availability. Supply chain issues make it difficult for many companies to maintain inventory.  

Complexity. It's possible to start from scratch and design all the necessary components, but you risk unnecessary and complicated development cycles and added expense. 

Total cost of ownership. Some products require bringing your own SIM card, adding to monthly recurring expenditures for connectivity and data. 

How to Build a Custom IoT-enabled PLC 

Think of PLC connectivity as having two ends: one end is the PLC, and the other end is the control system typically represented as a cloud application or database. In some workloads, like a function to report on PLC state condition, the PLC is a data producer, and the cloud system is the data consumer. In others, like a function to reconfigure a PLC remotely, the cloud system is the data producer, and the PLC is the data consumer. 

In either scenario, there are common problems that must be solved: 

• How will the PLC communicate with the wireless module? 
• Where will the wireless module send data? 
• How will the data be routed to your cloud application or database? 
• What is the security model used for network transport?

Wi-Fi vs. Cellular Connectivity for Wireless PLCs

When it comes to connectivity, the networking options can feel overwhelming. Every PLC application has its own requirements, so let's review the pros and cons at a high level to see which option is best for your project. The table below provides a quick comparison of connectivity based on the most common project considerations.

Why Choose Wi-Fi 

When a secure, managed Wi-Fi network is available, it can be an excellent connectivity option for the following reasons: 

• No additional data fees. Wi-Fi is often a better option because there are no cost limitations on the amount of data transferred. 
• Supports high bandwidth requirements. For streaming video or sending large amounts of data, Wi-Fi will be able to manage the transfer. 
• Connect in areas without cellular. You might have a facility in an area with poor cellular connectivity. In this case Wi-Fi could be your only option. 

When Wi-Fi is Not the Right Choice 

While Wi-Fi is often the first thought when considering wireless capability for PLCs, Wi-Fi connectivity has several critical drawbacks: 

• Wi-Fi networks may not be available. Either the PLC resides outside of a Wi-Fi umbrella, or the owner of the Wi-Fi network won’t let you use it because PLCs are a cybersecurity attack vector and Wi-Fi network operators seek to reduce their attack surface area. 
• Wi-Fi requires on-site configuration for each PLC. When deploying a Wi-Fi based PLC, each PLC must be configured on-site, which is time consuming and expensive. 
• Wi-Fi presents a security risk. IoT devices connected via Wi-Fi can be vulnerable to hackers if they don’t receive regular software support and system updates to address potential security issues. 

Why Choose Cellular 

If you need to report on conditions, configure PLCs, or remotely control PLCs, cellular should be evaluated for suitability. Here are the key benefits of cellular connectivity: 

• Reduced security risks. Using Cellular allows you to bring your own network to the system, giving you control and oversight over secure connectivity.  
• Always-on connectivity. The cellular network is ubiquitous and offers redundant capability, so outages are rare.
• Easy to set up. Cellular connectivity does not require extensive system set-up or ongoing maintenance. 

When Cellular is Not the Right Choice 

Not all workloads are a great fit for cellular, like if you need to stream video, or send high volumes of data. Here are some reasons cellular networking wouldn’t be the right choice: 

• High bandwidth requirements. Some PLCs stream data constantly, sending telemetry or real time data and these workloads are best suited for wired connectivity because of the required bandwidth. 
• Cellular dead spot. Cellular will not be an option if your geographic location doesn’t have a cell tower for miles or the physical environment blocks the signal. 

There are nuances to be aware of, so it might be helpful to do additional research comparing IoT connectivity options. The better you understand the capabilities and limitations of each connectivity option, the more confident you can be that you're making the right decision for your deployment.

Simply Retrofitting Your Legacy PLC with Cellular IoT 

There are other modems and modules on the market that can enable cellular connectivity with a legacy PLC.  However, when it comes to easily building a custom solution and finding the lowest total cost of ownership, the best way is to use a Blues Wireless Notecard. Blues Wireless benefits your business with: 

• Embedded SIM card with pre-paid data 
• Consumption-based pricing for routing and events 
• Contract-free, subscription-free service