Choose by the gap between your measured worst case and the published limit: keep 10 °C of thermal margin, demand IP6K9K for pressure washing, check the vibration report and the cranking dip, and confirm the CODESYS runtime version before you order.
Choose by the gap between what your machine does to the controller and what its datasheet allows. On farm machinery that usually means a mobile-machine controller: a metal housing rated around -40 to +85 °C, sealed connectors, a supply range that swallows the cranking dip, and a vibration report you can read. A cabinet-grade PLC keeps dying in this work because it was built for a clean, cool enclosure, and a harvester is neither. Five stresses decide the pick: heat, washing, vibration, supply dips and the CODESYS runtime. Each has a number you can measure and a limit you can demand, and this page gives you those numbers, the four realistic options, and a worked thermal example with the arithmetic shown.
What decides the choice
The margin decides, and you measure it yourself. Log the worst case at the mounting point, then demand a published limit at least one step beyond it.
| Stress | What you measure | What the datasheet must show |
|---|---|---|
| Heat | Temperature at the mounting point, logged across a full harvest shift | Operating range with headroom, plus a derating curve |
| Washing | How the machine is cleaned: hose, hot pressure washer, detergent | Ingress rating of housing and mated connectors, and approved cleaning conditions |
| Vibration | Where it mounts: cab wall, chassis, engine | Test report to IEC 60068-2-6 and IEC 60068-2-64, with the mounting arrangement |
| Supply | Voltage at the controller terminals during cranking and load switching | A range that covers dips to about 6 V on a 12 V machine and 10 V on a 24 V machine |
| Runtime | Your CODESYS version, libraries and retained data | Target package and runtime version for the exact order code |
Two planning rules run through the rest of this page. Temperature: keep at least 10 °C between the calculated enclosure interior and the lowest operating limit of anything in the box. Supply: the range must include the dip as measured at the terminals, because part of the drop lives in the harness.
What is failing on the current controller
Before you replace anything, name the stress that is winning. The symptoms separate cleanly.
- The controller stops or reboots as the machine warms up: heat. Log the temperature at the mounting point across a full shift and compare it with the rating.
- It reboots at key-on or when a big load switches: supply dip. Capture the voltage at the controller terminals; a capture at the battery hides the drop in the wiring, the terminals and the shared return.
- Green pins, corrosion or water in a connector after washing: the mated interface, a gland or a missing blanking plug. The housing rating was never the weak point.
- Intermittent inputs that track engine speed or travel: vibration and harness movement, at the bracket or where the loom rubs.
- A clean stop with a diagnostic entry: software or communications. Decode the record the way PLC fault code decoding for drive diagnostics does for a drive, then decide whether you need new hardware at all.
Lock out the machine before you probe anything. A voltage capture taken with the machine blocked gives the same reading, with none of the risk.
Controller options compared
Four classes cover the field. The limits below are the typical published ratings for each class; verify the exact model you are quoted.
| Option | Typical published limits | Where it wins | Where it loses |
|---|---|---|---|
| Cabinet PLC in an enclosure | 0 to 60 °C, vented enclosure, terminals inside the cabinet | Most I/O in the smallest space, and every integrator can program it | The cabinet becomes a second machine to engineer: cooling, gaskets, filters |
| Mobile-machine controller | About -40 to +85 °C, sealed housing, sealed circular connectors, supply range covering the cranking dip | Bolts onto the machine and washes with it | Fewer points per module, and fewer people on site who know the toolchain |
| Display-controller combination | Extreme-rated HMI and control in one sealed housing | One part does the cab display and the control | One failure takes both functions, and the face takes the sun and the wash |
| Distributed sealed I/O with a compact controller | Sealed blocks mounted at the sensors, one trunk cable | Short sensor runs, and most of the loom disappears | More parts to validate, and faults need the network view to find |
Washing: which ingress rating to demand
Match the rating to the wash pad, because that is the harshest water the controller will ever meet. On the common ingress scale, dust-tight, plain-jet and even 30 minute immersion ratings all sit below a wash-pad pressure washer, which runs close to 100 bar with water heated toward 80 °C. The rating that represents it is IP6K9K: high-pressure, high-temperature jetting from several angles. If the machine is pressure washed, demand IP6K9K on the housing and on every mated connector.
The rating belongs to the assembly. Housing, mating connectors, cable glands and blanking plugs all carry it, fitted the way the installation drawing shows. Fertilizer, detergent and salt sit outside every ingress test, so get the approved cleaning procedure in writing and stay inside the pressure and the standoff distance it names.
Heat: the margin rule
Two numbers decide heat: the dissipation inside the enclosure and the surface area that sheds it. For painted steel the standard planning figure is about 5.5 W per square metre per °C of rise. Aluminium sheds about twice that, stainless steel about two thirds. Keep 10 °C between the calculated interior and the lowest limit in the box.
Ask for the derating curve along with the range. Above the ambient the curve names, the allowed output current steps down so internal parts stay at or below their maximum ratings. A controller sized to its full current at 25 °C is a smaller controller at 50 °C.
Shade the cabinet. Midday sun puts about 1000 W per square metre onto a dark face, ground-level irradiance being what it is, and that can add more heat than the electronics inside it.
Example: sizing the thermal margin in a 500 x 400 x 200 mm cabinet at 40 °C
Inputs, each with its source:
- Ambient at the mounting point, logged across a harvest shift: 40 °C.
- Cabinet: painted steel, sealed, 0.5 x 0.4 x 0.2 m.
- Dissipation, summed from the datasheets: 12 W for the controller and 28 W for the DC/DC converter and interface relays, 40 W total.
- Lowest operating limit of anything in the box: 60 °C.
Rule in words: the interior temperature rise equals the heat inside divided by the surface area times the transfer factor for the enclosure material.
Arithmetic:
- Surface area: 2 x (0.5 x 0.4) + 2 x (0.5 x 0.2) + 2 x (0.4 x 0.2) = 0.40 + 0.20 + 0.16 = 0.76 m².
- Rise: 40 W / (5.5 W per m² per °C x 0.76 m²) = 40 / 4.18 = 9.6 °C.
- Interior: 40 °C + 9.6 °C = 49.6 °C, so call it 50 °C.
- Margin: 60 °C - 50 °C = 10 °C, exactly the minimum.
Decision: the box passes with nothing to spare. On a 45 °C afternoon the interior reaches 55 °C, the margin is 5 °C, and the rule fails. The cheapest fix is to move the DC/DC converter out of the enclosure: 12 W instead of 40 W gives 12 / 4.18 = 2.9 °C of rise, an interior of 48 °C at 45 °C ambient, and a 12 °C margin. The next fix is a cabinet with roughly double the surface area, which halves the rise. Ventilation comes last, because it opens a dust path back into the box. Below 10 °C of margin, change the box before you change the controller.
Supply dips and transients
Farm electrical systems dip hard at key-on. Cranking pulls a 12 V system down to about 6 V and a 24 V system to about 10 V, briefly, and a controller whose lower limit sits above that reboots every morning. Demand a range that covers the dip, or hold the supply up locally with a buffer module.
Measure at the controller terminals with a fast capture. A meter averaging on the battery reports the healthy end of a harness that is dropping volts at its far end. If the outputs switch inductive loads, follow the suppression the manufacturer requires for those channels: an output stage destroyed by a solenoid looks like an environment problem and is not one.
Vibration and mounting
Ask for the mechanical test report, in the standard the class uses: IEC 60068-2-6 for swept sine, IEC 60068-2-27 for shock, IEC 60068-2-64 for broadband random, and the road-vehicle mounting standard where it applies. The report is valid for the mounting arrangement it was tested with, so get that drawing too, with clamp positions and torque values, and copy it on the machine.
Chassis mounting needs more than a cab wall, and engine mounting needs more again. Most agricultural controllers belong on the chassis or the cab structure. Locking connectors and strain relief at the cable entry are part of the same qualification.
The CODESYS side of the choice
The programming environment is shared; the target is not. Libraries, I/O mapping, task configuration and retained data are specific to each controller, and a move from CODESYS 2.3 to 3.5 is a migration with real work in it. Get in writing the CODESYS version and the target package for the exact order code, and confirmation that the libraries your project uses exist for that target.
Plan the first commissioning day around the runtime. If the download lands and the runtime reports a different device, PLC runtime device mismatch recovery covers the safe path. Before anything else, checking whether a CODESYS runtime is running or stopped tells you which problem you actually have.
What to ask for with the quote
Send the vendor your measured worst cases and ask for these items:
- Operating temperature range with the derating curve and the installation restrictions it depends on: orientation, clearances, mounting plane.
- Ingress rating of the housing and of each mated connector, plus the approved cleaning conditions.
- Vibration and shock report with the mounting arrangement, and the standard it was run to.
- Supply voltage range and the cranking profile it covers.
- The CODESYS runtime version and target package for the exact order code, with library support confirmed.
- Connector and gland part numbers for the harness, because the harness sees the same water and vibration as the controller.
When to pick something else
Pick the cabinet PLC when the controller lives in a shaded, sealed cabinet inside a conditioned cab, the wash never reaches it, and the margin arithmetic above passes with room to spare. Pick distributed sealed I/O with a compact controller when the sensors are scattered and harness damage is the recurring failure; short runs to local blocks remove most of the loom. If the part that keeps failing is the display on a combined unit, split the functions so a cracked face costs a screen, not the control.
And if the underlying complaint is that nobody sees machine status, a notice screen that plays from a memory card with no network does that job on its own; shop-floor screens and notice boards covers where they go. Whether a screen like that should carry live process data or plain content is a separate choice, and live factory data display: signage or dashboard draws that line.
The decision
Recommendation: If the machine is pressure washed and works through harvest heat, buy a mobile-machine controller: an operating range around -40 to +85 °C, IP6K9K housing and mated connectors, a supply range that covers the 10 V cranking dip of a 24 V machine, and a mechanical test report for your mounting point. Confirm the enclosure arithmetic leaves at least 10 °C of margin and the target package matches your CODESYS project before the order goes in. Choose distributed sealed I/O when the harness is the failure point, and keep a cabinet PLC only inside a shaded, sealed cabinet that passes the same margin rule.
Questions
How much temperature margin should a PLC keep inside its enclosure?
Plan for at least 10 °C between the calculated interior temperature and the lowest limit of anything in the box. The margin covers logger placement error and the hot spots that sit above the average near power stages. Add the dissipation figures from the datasheets, divide by surface area times about 5.5 for painted steel, and add the rise to the logged ambient. Then check the derating curve, because allowed output current drops above the ambient it names.
Which ingress rating survives a pressure washer?
IP6K9K, tested with high-pressure jetting of roughly 80 °C water at about 100 bar. Dust-tight ratings, plain jet ratings and even a 30 minute soak at 1 m do not represent the wash pad. Demand the rating on the housing and on every mated connector, gland and blanking plug, and keep the washer inside the pressure and distance the cleaning procedure names.
Will a CODESYS 2.3 project run on a 3.5 controller?
It will need a migration, not a recompile. Libraries, I/O mapping, task configuration and retained data all move by hand between the versions, and a target-specific library is the usual blocker. Budget the migration as part of the hardware change, and get the runtime version plus the target package for the exact order code in writing before the purchase order.
Why measure supply voltage at the controller and not the battery?
Because the cranking dip arrives at the terminals smaller than it leaves the battery. Starting pulls a 12 V system down to about 6 V and a 24 V system to about 10 V, and the harness, the terminals and the shared return take their share on the way. A fast capture at the controller terminals is the only reading that shows what actually resets it.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.