ARM Industrial Control Board Design for Fanless Systems

An industrial controller that runs for years inside a cabinet has different priorities from a desktop computer. An arm industrial control board is built around those priorities: modest power, no moving parts and interfaces that survive being wired to a machine.

What the Board Is

An arm industrial control board is a computing module built around an ARM processor together with the interfaces a machine needs: Ethernet, USB, serial ports, CAN, RS485, digital input, display and storage. It is designed to be integrated rather than used as a general purpose computer.

That definition drives the whole design. The board has to boot reliably from a fixed image, run without a fan and survive continuous operation in a cabinet where the ambient temperature is not controlled by the user.

Where Low Power Comes From

ARM cores deliver useful performance per watt because the architecture was designed for battery powered devices. The result on a control board is a device that consumes a few watts rather than the tens of watts a comparable x86 board would draw.

Low power translates into low heat, and low heat is what allows the board to be cooled without moving air. A processor running at a small fraction of a desktop part can be cooled by conduction into the enclosure instead of by a fan.

Fanless Operation

Removing the fan removes the least reliable component in the system. It also removes the airflow that carried dust and conductive particles into the enclosure, which is a common cause of field failures in industrial equipment.

The trade is that heat has to leave through the board and the case. That path is designed rather than assumed: copper area under the processor, thermal vias to the far side and a defined contact area with the enclosure wall.

Industrial Interfaces

Field wiring is where a control board meets the real world. Serial ports, CAN and RS485 transceivers need isolation, common mode range and protection that a laboratory interface simply does not require.

Every industrial interface is also a grounding decision. Isolated ports break the ground loop between the machine and the controller, and the isolation barrier has to be crossed by the power supply as well as by the data lines.

Power Supply and Wide Input

Industrial rails are not clean. A 24 V supply can swing widely and it carries the transients produced by contactors opening and motors starting. The input stage has to tolerate that without allowing the processor to reset.

A wide input regulator followed by point of load converters is the usual arrangement. Sequencing matters, because the processor core, the memory and the I/O rails have to come up in a defined order and reach regulation before the reset is released. The layout of those stages follows the rules in our converter layout notes.

Storage and Boot

ARM boards typically boot from managed NAND, eMMC or a SPI flash device. The boot medium determines the update strategy as much as the storage size determines the capacity of the product.

An industrial product needs a recovery path. A second boot image, a serial download mode or a removable medium allows a unit that fails during an update to be recovered on site instead of being returned to the factory.

Data Security

Data at rest and data in transit both matter once the controller sits on a plant network. Secure boot verifies the image before it is executed, and encrypted storage protects the configuration and any credentials held on the board.

Because the flash is soldered down, a casual attacker cannot simply remove the medium. That is a real advantage over a system built on a removable drive, provided the debug ports are closed before the product ships.

Shock and Vibration

Industrial equipment is installed on machines that move. Solder joints are the first thing to suffer, so the mechanical design of the board matters as much as the electrical design of the circuit on it.

Heavier parts need anchoring, connectors need strain relief, and the board should be fixed at enough points to keep its first bending mode well above the vibration produced by the machine it controls.

Connectors and Field Wiring

Field connectors are chosen for the wiring practice of the customer rather than for the convenience of the layout. Terminal blocks, pluggable headers and D sub connectors each impose their own footprint and keep out requirements.

Label every pin on the silkscreen and in the documentation. A mis-wired field connection is one of the few faults that can destroy a board which is otherwise correctly designed and built.

Bring Up and Debug Access

Keep a serial console, a JTAG header and a few status LEDs on the first build even when the production version will not need them. Bring up is faster when the board can report what it is doing.

Test points on the supply rails, the reset line and the clock turn an intermittent fault into a measurement. The pads cost almost nothing and they are impossible to add once the panels have been ordered.

Long Term Availability

Industrial products stay in service for a decade or more, which is longer than the commercial life of most processors. Choose a part on a long life programme and design so that the memory and the power stage can be re-sourced.

Keep the parts that are hard to replace on the outside of the design where possible. A processor module or a socketed memory device is easier to migrate than a fine pitch package available from a single supplier.

Thermal Design of the Enclosure

Thermal design does not end at the board edge. The enclosure, the mounting arrangement and the position of the unit inside the cabinet all change the case temperature that the processor will actually see.

Worst case calculation should use the maximum ambient the product is rated for plus the temperature rise of the cabinet, and the result should be verified on a prototype in the real installation rather than on a bench.

Layout Practice

Route the memory bus as a group with matched lengths and a continuous reference, and keep the switching supplies away from any analogue front end. A processor data sheet is a starting point, not a complete layout guide.

Use copper to move heat as well as current. The plane under the processor is part of the thermal design, and the vias that connect it to the opposite side are part of the same decision. Current density in those paths is checked with the same table used for any trace width calculation.

A Short Checklist

Before release, confirm supply sequencing, reset timing, boot configuration straps and every isolation barrier. Check that each external interface has protection and that the case temperature stays inside the rating of all fitted parts.

Then check the documentation. A control board is installed by someone else, so the pinout, the connector types and the wiring diagram are part of the product. Moisture and condensation deserve a thought as well, and the measures in our conformal coating notes are a reasonable default for a sealed cabinet.

FAQ

Why not simply add a fan? Because a fan is a wear item with a finite life, it draws dust into the enclosure and it adds audible noise. Conduction cooling through the case removes all three problems at the cost of careful thermal design.

Is an ARM processor fast enough for control work? For most machine control, measurement and gateway tasks it is, and the low latency of a dedicated core often matters more than peak throughput. Choose the part from the interface count and the software stack.

How should the board be protected against transients on the 24 V rail? With a fused input, reverse polarity protection, a transient suppressor and a wide input regulator that tolerates the highest surge the installation can produce.