What to Look for in a Controller Supplier: A PLC and Drive Specification Checklist

This is for engineers and buyers who are evaluating PLC and drive suppliers and want a practical pre-order check. I'm a controls engineer who has been specifying automation components for custom machines for 8 years. In that time, I've personally made and documented seven specification mistakes that added up to roughly $7,200 in wasted budget and debugging time. This checklist is the one I now run before I approve any controller supplier.

It is not a design manual. It is a screen. There are five checks, and they work best in this order.

Check 1: Define the machine before you compare PLC specifications

Starting with processor memory and scan time feels logical, but it can hide the kind of mistake I made in 2019. I specified a Delta PLC DVP unit for a small bottle-sorting line. The CPU had enough inputs and outputs, but the application needed more high-speed counting channels and serial ports than that CPU supported. The processor looked fine on paper, the I/O count looked fine, and the bench test looked fine. At full line speed, the counts drifted and the reject timing moved.

The issue was not the CPU brand. The issue was the order of thinking. I compared PLC specifications before I had documented the machine behavior. Write a functional description first: sensors, actuators, protocols, HMI tags, motion axes, alarm states, and operator modes. Then convert that into a hardware spec. The CPU model is a result, not a starting point. That mistake cost roughly $800 in replacement hardware plus a weekend of rework.

Check 2: Put the main PLC specifications in one comparison table

Once the functional spec exists, create one table and compare the same line items for each controller candidate. At minimum, include:

• Total digital I/O, including expansion limits
• Transistor output versus relay output availability
• Program memory and retentive data memory
• Execution speed and scan time
• Built-in serial and Ethernet ports plus supported protocols
• High-speed counters and hardware pulse outputs
• Common terminal current per output group
• Ambient temperature rating and mounting orientation

This step feels administrative, but it catches gaps that headline spec comparisons do not show. I remember a project where every candidate CPU had enough points, and only one had enough built-in pulse outputs for the stepper axis we planned. That difference would have showed up after wiring if we had skipped the table.

I also use the table to explain tradeoffs to the customer. I would rather spend ten minutes walking through a specification than manage an unhappy surprise after start-up. In my view, that is what customer education should look like: give people a way to compare, not a reason to be confused.

Check 3: Check the specifications most people skip

Here is the counterintuitive one: a PLC with enough I/O points is not fully specified. Every output point has a rated current, and every output group usually has a common terminal that carries the combined return current. If you group several outputs onto one common, the individual points can each be under their rating while the common connection is over its rating.

That happened to me in September 2022. Six output points were powered through one common terminal. No breaker tripped, and no output point was overloaded by itself, but the common terminal had to carry more current than its rating. It ran hot enough to smell. The data sheet showed the common terminal rating; I just had not looked for it. Rewiring cost about $1,100 plus a lost production day.

Also check the ambient temperature rating. Compact PLC and drive products publish thermal ratings under IEC 61131-2 test conditions. Those conditions rarely include a sealed cabinet beside a heat-generating drive. I now estimate panel temperature before I approve the controller, not after the first thermal fault.

Check 4: Read drive specifications with the motor attached

Drive selection is not just horsepower matching. The drive data sheet lists output current at a given switching frequency and an overload curve. I learned this the hard way on a conveyor in 2023. We set the switching frequency high to reduce motor noise, then ran the machine at full load. The drive went into over-temperature after about 40 minutes. The drive current rating at the default switching frequency was fine; the rating at the selected frequency was not.

The drive specification that matters is continuous output current at your switching frequency, ambient temperature, and overload duty. Compare that to the full-load current on the motor nameplate. If the motor runs at low speed for extended periods, look at the constant torque rating and decide whether the drive needs a larger frame or external cooling. Also confirm whether a braking transistor is built in if the load must stop quickly; a braking resistor is easy to add later, but a drive without the braking stage is not.

Check 5: Investigate the controller supplier, not only the catalog

What to look for in a controller supplier changed for me after several years of panel building. I used to believe that the supplier with the longest feature list was the safest choice. Now I ask how that supplier behaves after the quote.

• Can I download the PLC programming software and documentation before buying?
• Are CAD files, network descriptor files, and sample programs available without a special request?
• Does the local distributor stock the CPU and I/O modules I am specifying?
• What is the real lead time this quarter? (Should mention: check lead time more than once a year because supplier logistics change.)
• What is the process for a dead-on-arrival or incorrect part? Is it advance replacement or an RMA first?
• Is this series active and supported, or close to end-of-life?

This is where a broad product range and a distributor network matter. We use the Delta PLC DVP range in several small machines because the local distributor can answer technical questions and stock common modules. (Mental note: distributor response time should be tested before you need it, not during a breakdown.)

In my experience, a supplier that publishes clear specifications and treats spec questions seriously is also more likely to help when a machine does something unexpected. An informed customer asks sharper questions and makes faster decisions.

Final reminders before you approve the order

The mistakes I document now live in our onboarding folder. These are the reminders I repeat most often:

• Relay outputs cannot generate fast pulse trains. If a stepper or servo drive needs pulse commands, pick a transistor output with the required speed.
• Spare capacity is not just empty I/O points. Leave spare communication channels, high-speed channels, and memory for future revisions.
• Verify common terminal current and panel ambient temperature in the real enclosure design, not only on the product data sheet.
• Use the same specification table for every supplier; otherwise you are comparing marketing summaries, not technical documents.
• Ask about product lifecycle and supplier support before you need them.

No checklist can replace careful engineering. It can, however, turn your past mistakes into a process that keeps the next order from becoming another expensive lesson.

Rebecca Sloan

Rebecca Sloan is a power distribution and protection analyst specializing in circuit breakers, switchgear, contactors, fuses, surge protective devices, and coordination. She applies IEC 60947-2 breaker requirements, IEC 60269 fuse characteristics, and IEC 61643-11 tests while examining rated voltage, breaking capacity, time-current curves, selectivity, and prospective short-circuit current. She helps engineers and buyers compare protective devices against documented fault levels, installation conditions, maintenance access, and continuity priorities.