12 Pre-Purchase Checks Before Ordering a New Industrial PLC

1. Define the Exact Control Scope Before Looking at Catalogs

The first step in PLC selection is writing down what the machine or line must actually do. A vague brief like “we need a bigger PLC” leads to overbuying or underbuying. List every input, output, analog signal, and digital event the controller must handle. Count the exact number of digital inputs and outputs required for the current machine. Then add a margin for future expansion. A common mistake is buying a base unit with no spare slots. If the line grows by two sensors next year, you may need a whole new backplane or an extra chassis.

Do not just count the terminals. Check the electrical load on each output. A relay output can switch a motor, a solenoid, or a contactor. A transistor output may handle a small heater or a valve. The load type changes the internal design and the heat the module generates. If the environment is hot or dusty, the thermal margin of the cabinet matters as much as the part number.

2. Validate I/O Counts and Expansion Slots

This is the core of controller sizing. The number of I/O channels must match the process. A small packaging line may need a few tens of channels. A complex mixing plant may need hundreds. The base unit is often just a controller and a small number of I/Os. The rest of the system is built by adding I/O modules to a backplane or a fieldbus.

Check the maximum number of modules the platform supports. Some platforms cap at a certain number of slots. If you need more modules than the platform allows, you must split the system into two controllers or use a different family. This is a hard limit that cannot be fixed later with a software update.

Look at the pin counts for each module type. A 16-channel digital input module and a 32-channel digital input module do not always fit in the same slot. Some platforms use standard slot widths. Others use wide slots for analog or high-speed I/O. Mixing and matching modules without checking the slot compatibility can waste time on the factory floor.

12-Point PLC Pre-Purchase Checklist

Use this numbered checklist to validate the controller before you release the purchase order.

1. I/O Count: Verify the total number of digital inputs, digital outputs, and analog channels. Add a margin for future growth.
2. Slot Capacity: Confirm the maximum number of modules the backplane supports.
3. Power Supply: Check the internal power supply voltage and current rating. Verify it can handle all modules simultaneously.
4. Redundancy: Determine if hot-swap power or redundant controller cards are required for uptime.
5. Communication Protocols: List the required fieldbus and Ethernet protocols. Confirm the controller supports them natively.
6. Software Licensing: Check if the engineering software is included in the price or costs extra.
7. Spare Parts: Confirm availability of spare I/O modules, power supplies, and controller cards.
8. Service Response: Ask about the vendor’s response time for hardware failures and software support.
9. Cabinet Thermal Load: Calculate the heat generated by the modules and verify the cabinet can dissipate it.
10. Environmental Rating: Check the IP rating and operating temperature range against the site conditions.
11. Integration Costs: Estimate the cost of cabling, programming, and testing.
12. Vendor Lock-In: Review the terms for source code access and future software updates.

3. Check Power Supply and Redundancy

The power supply is the heart of the PLC. Most industrial controllers use a 24 V DC supply. Some use 115 V AC or 220 V AC. The supply must handle the inrush current of all connected modules. If you add a large number of analog modules, the power draw increases. If the supply is undersized, the system may reset during high load.

Redundancy is a commercial decision, not just a technical one. For critical lines, a hot-swap power supply allows you to replace a failing unit without stopping the machine. For less critical lines, a standard supply is often enough. Ask the vendor about the mean time between failures for the power supply. If the supply fails, the whole controller goes down. A redundant setup protects against this single point of failure.

Check the power connections. Is it a terminal block or a plug? Terminal blocks are more secure for vibration. Plugs are easier for maintenance. Some platforms allow external power supplies. This adds flexibility but requires extra wiring and a dedicated enclosure.

4. Confirm Communication Protocols and Network Integration

A PLC is not an island. It talks to HMI screens, SCADA systems, and other controllers. The communication protocol determines how fast and how reliably data moves. Ethernet-based protocols are common in modern plants. Fieldbus protocols are still used in older or specialized lines.

Verify that the controller has a native support for the required protocol. Some protocols require a separate gateway or a specific communication card. This adds cost and complexity. If you are moving from an old fieldbus to a new Ethernet standard, you must check if the new controller can talk to both during the transition. This is a common pitfall in PLC selection.

Look at the data rate. A simple status flag may not need high speed. A closed-loop control may need millisecond response times. The internal scan time of the controller matters here. A fast scan time reduces the delay between reading inputs and writing outputs. If the process is fast, a slow scan time can cause instability.

5. Review Software Licensing and Engineering Tools

The price of the controller is only part of the cost. The engineering software is often a separate license. Some vendors include a basic runtime in the hardware. Others charge for the full engineering suite. Check if the license is per-seat or per-controller. If you have multiple engineers, the cost adds up.

Check the software compatibility with your operating system. Some older tools run on specific Windows versions. Newer tools may require 64-bit systems. If your plant standard is a specific OS, make sure the software supports it.

Look at the online and offline engineering capabilities. Offline engineering is done on a PC. Online engineering is done on the machine itself. For small changes, online editing is faster. For complex logic, offline engineering is safer. Some platforms allow both. Others only allow one. This affects how your team works day to day.

6. Assess Spare Parts and Service Support

A PLC selection is a multi-year commitment. If the controller fails, you need a spare part quickly. Check the vendor’s spare parts program. Are spare I/O modules and controller cards in stock? What is the lead time? A two-week lead time can be acceptable for a non-critical line. It is not acceptable for a bottleneck machine.

Ask about the service level agreement. What is the response time for a hardware failure? Is there a next-business-day replacement? Some vendors offer on-site support for critical assets. Others only offer remote support. This depends on your risk tolerance.

Check the warranty terms. Standard warranties are often one to two years. Extended warranties cost extra. If the controller is part of a larger system, the warranty may be tied to the system, not the individual part. Read the fine print.

7. Calculate Total Cost of Ownership

The sticker price is the starting point. The total cost of ownership includes the controller, I/O modules, power supplies, software licenses, cabling, and integration labor. A cheaper controller can become expensive if it requires more modules or more labor to install.

Consider the cost of downtime. If the controller fails, what is the cost per hour of production loss? A more expensive, redundant controller may pay for itself in avoided downtime. This is a common trade-off in controller sizing.

Check the cost of future upgrades. If the platform allows you to add modules without replacing the base unit, the long-term cost is lower. If every expansion requires a new controller, the lifecycle cost is higher. Ask the vendor for a sample bill of materials for the full system. This helps you compare apples to apples.

8. Align with Existing PLC vs IPC Strategies

Many plants use a mix of PLCs and Industrial PCs. A PLC is best for fast, deterministic control. An IPC is best for complex logic, data handling, and HMI. Do not force a PLC to do what an IPC does well. Do not force an IPC to do what a PLC does well.

If your plant already uses a specific PLC family, sticking with that family can reduce training costs and spare parts inventory. If you are moving to a new standard, you may face a learning curve. The PLC selection should fit the broader automation strategy. A mismatch can create a fragmented plant with different tools, protocols, and maintenance workflows.

Check if the PLC can integrate with the existing IPCs. If the IPC handles the recipe management and the PLC handles the motor control, the two must talk to each other. The communication protocol and the data structure must be defined early. This prevents integration problems later.

9. Verify Environmental and Mechanical Fit

The physical fit is often overlooked. Check the dimensions of the base unit and the I/O modules. Do they fit in the existing control cabinet? The cabinet may have limited space. A larger controller may require a new cabinet.

Check the mounting options. Some controllers are DIN-rail mounted. Others are chassis-mounted. The mounting affects the wiring and the access for maintenance. If the cabinet is already full, you may need to add a new panel.

Check the environmental rating. A standard PLC may have an IP20 rating. If the cabinet is in a dusty or wet area, you may need an IP65 rated cabinet or a sealed enclosure. The IP rating of the controller itself is less important than the IP rating of the cabinet, but it still matters.

10. Plan for Maintenance and Operator Training

The PLC selection should consider who will maintain it. If your team has experience with a specific platform, the training cost is lower. If you are switching to a new platform, budget for training time. The engineering software may have a steep learning curve.

Check the documentation. Are the manuals available online or only as PDFs? Are there online forums or knowledge bases? Good documentation reduces troubleshooting time.

Check the operator interface. If the HMI is built on the same platform as the PLC, the integration is smoother. If the HMI is from a different vendor, you may face compatibility issues. The operator experience matters too. If the interface is confusing, operators may make mistakes.

11. Check Regulatory and Safety Compliance

The PLC must meet the safety requirements of the process. If the machine handles hazardous materials, the controller may need to meet specific safety standards. A standard PLC is not a safety controller. For safety-critical functions, you may need a safety-rated controller or a separate safety system.

Check the certification. Does the controller have the required certifications for your region? Some regions require specific safety marks. The vendor should be able to provide the certificate.

Check the electrical compliance. The wiring and the power supply must meet local electrical codes. The PLC itself may be compliant, but the installation may not be. This is the responsibility of the integrator, but the controller choice affects the wiring design.

12. Review Vendor Lock-In and Future Proofing

Vendor lock-in is a real risk. If the vendor goes out of business, you may be stuck with obsolete hardware. Check the vendor’s financial health and their long-term support plan.

Ask about source code access. If you need to maintain the code after the vendor support ends, do you have the source code? Some vendors offer source code escrow. This protects you if the vendor disappears.

Ask about the roadmap. Will the vendor support this platform for the next five years? Will they provide security updates for the Ethernet stack? A controller that is not updated may become a security risk. The PLC selection should consider the long-term viability of the platform.

13. Finalize the Commercial Terms

The technical checks are done. Now check the commercial terms. The price should include the base unit, the I/O modules, the power supply, and the software license. If the software is extra, add it to the quote.

Check the payment terms. Net 30, Net 60, or advance payment? Some vendors require a deposit for custom configurations. The payment terms should match your cash flow.

Check the delivery time. A standard PLC may ship in two weeks. A custom configuration may take two months. If the project has a hard deadline, order early. The delivery time is often the bottleneck in PLC selection.

Check the return policy. If the controller arrives with a defect, what is the return process? Some vendors offer a simple return. Others require you to ship it back with a tracking number. The return policy should be clear.

14. Make the Decision and Document the Rationale

Once the checks are done, document the rationale for the selection. Write down why this platform was chosen over alternatives. This document helps the maintenance team in the future. It also helps the procurement manager if a similar need arises.

The decision should be signed off by the engineering lead and the procurement manager. Both parties must agree on the technical specs and the commercial terms. This prevents disputes later.

The PLC selection is not a one-time event. As the plant changes, the controller may need to be reviewed. A checklist can be reused for future projects. The goal is to make the selection systematic, not ad hoc.

A well-chosen PLC runs quietly for years. A poorly chosen PLC causes headaches for years. The 12-point checklist is a simple tool to prevent the latter. Use it every time you order a new controller.