Published: 28.05.2021 | Edited: 31.05.2021 | Tags: 100daystooffload,electronics,arduino,plc,industrial

M-Duino external voltage reference trap

When wiring the Arduino based Programmable Logic Controller (PLC) from Industrial Shields, I have become stuck for a little bit due to trouble understanding how to enable the outputs.

Currently using their Ethernet range based on Arduino Mega 2560. Its internals seem to be quite quite good quality, although it is hard to disassemble. It is even harder to assemble it back so I just had a peek inside to asses the overall quality. Also the teardown can already be seen in this YouTube video, so no much need replicating that.

This product line of Industrial Shields are based on standard Arduino boards, which are themselves Open Source Software (OSS) and Open Source Hardware (OSHW), but there are additional boards that are not OSHW. It is understandable as they have to turn in profit somewhere.

Output voltage reference

As can be seen in the data sheet, for example for model AR38+, the outputs are Digitally Isolated Outputs. This is confirmed by the Peter's video above. The data sheet further mentions that the voltage range for the Digitally Isolated Outputs is 5 to 24 Vdc. This all implies that the output voltage can be set.

But how to set the output voltage for the Digitally Isolated Output pins, specifically Q0.0 to to Q0.4? PWM pins are affected too, by the way. Well there is a pin labeled Q/Vdc and it's description states:

Voltage Supply/Reference for Digital/PWM Outputs (isolated)

The important bit is that the rest documentation is little bit lacking in respect of output voltage. Although it is mentioned this pin exists, I could not find any mention that is required to be connected, otherwise the output voltage will be zero, even when the output LED is ON on the front plate. So remember to connect the Q/Vdc pin to 5V, 12V or 24V on M-Duino.

Update 31.05.2021

Do not forget to connect COM(-) pin to ground. It is galvanically isolated too, so connect it either to a common ground or a separate one, depending on the application. I have left it floating thinking that I only need the high reference and it made the related Q pins rise above the 0V when in LOW state, such that they were considered HIGH by the receiving end. This is is something to keep in mind - properly connect both references when using digital outputs.

This is a 79th post of #100daystooffload.