Retrofitting a legacy machine with an Arduino PLC usually comes down to one practical question: does the application need onboard Ethernet and a higher I/O count, or does it need to switch loads directly in a compact footprint? M-Duino and Ardbox answer that differently, and the right pick depends on the machine, not on which family is better overall.
Why a retrofit narrows the choice quickly
A retrofit project rarely starts with a blank panel. There is usually limited space, existing wiring, and a machine that either has no data output at all or one that stops at a local display. Adding an Arduino PLC means fitting it into that existing panel, wiring it to sensors and actuators that were never meant to talk to a network, and deciding how the data leaves the machine, if it needs to leave at all.
That context, more than any general spec sheet, is what should drive the choice between M-Duino and Ardbox.
M-Duino: more I/O and onboard Ethernet
M-Duino is based on Arduino Mega. The 21 model, for example, provides 13 inputs, 6 analog/digital plus 7 digital isolated, and 8 outputs, 5 digital isolated plus 3 digital/PWM/analog, with onboard Ethernet, RS-485 and RS-232 as standard. Larger M-Duino models extend the I/O count further.
Onboard Ethernet matters in a retrofit when the plan is to feed machine data into a SCADA system, a historian or an MES over Modbus TCP, without adding a separate gateway. If the retrofit is primarily about data acquisition and monitoring, and there is room in the panel for a slightly larger controller, M-Duino is usually the better fit.
Ardbox: compact and built to switch loads directly
Ardbox is based on Arduino Leonardo, in a smaller DIN rail footprint than M-Duino. It comes in two variants. Ardbox Analog provides 10 inputs, 8 convertible analog/digital plus 2 digital isolated, and 10 outputs, 3 digital isolated plus 7 digital/PWM/analog. Ardbox Relay provides 10 inputs, 6 convertible analog/digital plus 4 digital isolated, and 10 outputs, 2 digital/analog plus 8 relay outputs rated at 220 Vac, 5 A. Neither variant has onboard Ethernet.
The Relay variant is the one that matters most for retrofitting: it switches pumps, motors or lighting directly, without adding an external relay module to the panel. Combined with its smaller size, Ardbox tends to fit retrofits where panel space is tight and the main job is switching or reading a modest number of signals, not feeding a network.
Same Arduino IDE, different retrofit fit, not a difference in quality
Both families are programmed with the standard Arduino IDE, using the Industrial Shields board package to map industrial I/O pins to Arduino pin names. Neither is a lesser version of the other. M-Duino trades a larger footprint for more I/O and native network connectivity. Ardbox trades onboard Ethernet for a smaller size and, in the Relay variant, direct load switching.
| If the retrofit needs | Pick |
|---|---|
| Onboard Ethernet for SCADA/MES integration | M-Duino |
| A higher I/O count in one controller | M-Duino |
| The smallest possible footprint in a tight panel | Ardbox |
| Direct relay switching of pumps, motors or lighting | Ardbox Relay |
| Precise analog signal monitoring in a compact unit | Ardbox Analog |
Compare the full range before deciding
For the complete specification table, including the applications each family is typically used for, see our Arduino PLC comparison page.
If the retrofit needs native WiFi instead, take a look at the ESP32 PLC range.
If it needs a Linux environment for Node-RED and SCADA, take a look at the Raspberry PLC range.