The 6-Point PLC and HMI Ordering Checklist I Wish I Had in 2021

Who This Checklist Is For

If you're a solar installer or system integrator who orders PLCs and HMIs for your projects, this is for you. Specifically, this checklist applies when you're moving from a basic relay logic setup to a more sophisticated programmable controller system, or when you're placing a repeat order for a control cabinet you've built before.

I handle procurement and technical specs for a mid-sized solar integration firm. Over the past four years, I've personally placed orders for over 200 PLC and HMI units. And I've made some expensive mistakes. This 6-point checklist is born from those errors.

The goal is simple: a quick sanity check before you hit 'submit' on that order form.

Step 1: Verify Your HMI Resolution and Protocol Match

When you order an HMI, it's tempting to just match the physical size. But the real gotcha is the communication protocol. I once ordered a 7-inch HMI that looked perfect on paper. It had the right serial port, the right power supply. What I missed? It only spoke Modbus RTU, and our PLC was using Modbus TCP. The unit had to go back. A $450 mistake plus a 1-week delay.

Check this: specify both the protocol (TCP, RTU, Profibus, etc.) and the port type (RJ45, DB9, USB). Also, don't assume the HMI's demo project will look the same on the final machine. The resolution (800x480 vs 1024x600) can shift your carefully designed buttons around. Get the exact model number with revision letter.

Step 2: Don't Assume the Relay Module's Coil Voltage

This is a classic. People think 'relay module' is a one-size-fits-all component. It's not. The big split is between the coil voltage and the contact rating.

On a 32-piece order where every single unit had to have a specific 24V DC coil relay module, I checked the form factor, checked the contact rating (10A at 250V AC — fine), but I never double-checked the coil voltage. The supplier shipped units with 24V AC coils. They looked identical. The wiring was already done. We caught it only during commissioning. That error cost $890 in redo plus a 1-week delay (this was back in September 2022).

So: make a line item for 'Coil Voltage: DC/AC' and 'Contact Voltage/Current' on your bill of materials. It's a no-brainer, but it's the first thing to slip when you're in a hurry.

Step 3: Ladder Diagram PLC — Are You Buying the Right 'Flavor'?

There's a misconception here. Some people think a 'ladder diagram PLC' is a specific type of hardware. Actually, it's a programming standard. Most modern PLCs can be programmed in ladder logic, but the hardware itself might be optimized for something else.

The assumption is that if you buy a PLC from a brand like Siemens or Allen-Bradley, you're getting a 'ladder diagram PLC.' The reality is that the choice of CPU model determines the instruction set you have available. A smaller CPU might not support advanced math or data manipulation instructions you need for a complex sequence. I've seen integrators buy a cheap compact PLC only to find out it can't handle the looping logic required for a multi-state solar tracker controller. The result: a 3-day production delay while we swapped out the CPU. That's what happens when you treat 'ladder diagram' as a feature rather than a programming language.

Check your CPU's instruction set manual before you order.

Step 4: Transistor Output PLC vs. Relay Output — It's Not Just About Switching Frequency

A lot of people know the textbook answer: 'Use transistor outputs for high-speed switching (like stepper motors) and relay outputs for high-current loads.' That's true. But the mistake I made was assuming that was the only consideration.

The real gotcha is leakage current. A transistor output (especially a sinking type) can have a small leakage current even when 'off.' I ordered a transistor output PLC for a solar pumping application where we needed to control a small hydraulic solenoid valve. The valve was 24V DC, low power. The transistor output 'off' state still had a few milliamps of leakage. The solenoid never fully closed. It just buzzed. We chased that ghost for two days. $320 in wasted labor, credibility damaged. Lesson learned: for low-power inductive loads when 'off' must mean 'off,' a relay output is safer. The transistor output is great for speed, but the leakage current is a deal-breaker for certain loads.

So, before you order, ask: 'What is the off-state leakage current of this output module?'

Step 5: Analog Inputs Module — Don't Forget About Resolution and Sample Rate

Ordering an analog inputs module seems simple: 'I need 4-20 mA, 0-10V, or thermocouple.' But the nuance is in the resolution and sample rate.

People think a 12-bit resolution input is the same as a 16-bit one, just maybe a bit less accurate. Actually, the difference in precision can be the difference between your solar panel tracking system pointing at the sun or missing it by a degree.

I once ordered a 4-channel analog input module (circa 2021) for measuring pyranometer (solar irradiance) data. The device said '4-20 mA input.' I checked the price, ordered it. But the resolution was only 12-bit over a 0-20 mA range. For our application, the difference between 800 W/m² and 805 W/m² was meaningful for our performance modeling. The 12-bit module simply couldn't resolve that. The data was noisy. We ended up using a 16-bit module with a faster filter. The $50 difference per project translated to noticeably better data quality.

On your order, spec the bit resolution and the update rate (sample time). For a solar tracker or a BMS system, 16-bit is pretty standard. For simple level sensing, 12-bit might be fine.

Step 6: Analog Outputs Module — The Zero-Scale/Bias Trap

The analog outputs module is where I made my most embarrassing mistake. I ordered a 4-channel analog output for controlling variable speed drives (VFDs) on our coolant pumps. The spec said '0-10V output.' I assumed that meant a 0V signal = pump stopped, 10V = pump full speed.

But some analog output modules have a 'zero-scale' or 'bias' setting. This is a configuration parameter in the module's software or DIP switches. On a $3,200 order of modules, I just dropped them in without checking this. We commissioned the system, and at a 0V command, the VFD was still running at 5% speed. The pump was barely turning. The VFD wasn't faulty—the analog output module had a factory default bias of 0.5V. That meant the 'analog 0' was actually 0.5V to the VFD. Adding to the confusion, the 10V output was correct. So the pump wouldn't stop. That took an hour of head-scratching to diagnose.

Always verify the zero-scale setting on your analog output module. Some are configurable for 0-10V, 2-10V, or 4-20 mA. Don't assume the default is the 'clean' 0V.

Common Mistakes and Notes

• Mixing sourcing and sinking: A transistor output PLC (sourcing) usually provides a positive voltage when on. A sinking input module needs current to flow into it. If you wire a sourcing output to a sinking input without a pull-up resistor, it won't work. This is a common red flag for first-time integrators.
• Ordering too few spare channels: You'll always need one more analog input or one more relay output than you planned. Add 20% to your I/O count for future expansion or troubleshooting. That's a rule of thumb I've learned the hard way.
• Ignoring the HMI's software license: The HMI hardware is cheap. The software license (the tag database, runtime license) can be the expensive part. I once ordered two HMIs but only one software license. We had to pay $200 for a second license overnight. A ballpark cost that I didn't budget for.

The bottom line: quality on these components directly affects your system's reliability and your client's perception of your work. A $50 difference on an analog module can save a $3,000 site visit. I think spending the extra time on this checklist is a no-brainer. It's fairly straightforward once you know to check these six points.