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My company is developing and producing measurement devices. To quality check and adjust the full devices automatically, we need to be able to read the display indication and trigger functions like high resolution, zeroing, etc.

We are using usually the built-in communication interface (RS-232, WiFi, Ethernet). However, cheaper variants do not have a communication interface except the buttons and LCD display for the customer.

Reading would be possible with OCR, but pressing the various buttons with a robot hand (morse code or similar) is even more "complicated".

To keep product cost low, we are thinking of having a more or less hidden connector with a MCU UART connected to it directly.

Personally, I'm wondering if it would be even cheaper and more convenient to connect a processor pin to a medium sized tiny PCB antenna. Then try to use an UART or bit-banging to "radio" out signals, basically to intentionally cause low-frequency EMI. Outside of the device under test, there is a very close and big antenna for receiving the signal. I assume trigger functions in the device is the harder part.

Is this a viable idea? Can you point me to a term or solution I can further research? Thank you very much!

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    \$\begingroup\$ Apologies if you have already, but have you considered using IR to communicate? It solves both the low cost and connector issue, whilst being reliable and simple to set up. Such an LED could be hidden in plain sight, behind a translucent bezel around the LCD - should it have one. \$\endgroup\$
    – MIL-SPEC
    Jun 13, 2018 at 13:04
  • \$\begingroup\$ @User9123 please post that as an answer. I want to upvote you and refer to your answer in mine. \$\endgroup\$ Jun 13, 2018 at 15:09
  • \$\begingroup\$ @MarcusMüller I have posted it as an answer :) \$\endgroup\$
    – MIL-SPEC
    Jun 13, 2018 at 15:16

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IR as described by @User9123 is actually used in a lot of cheap game handhelds etc., but also with pocket calculators and so on.

Audio communication might work, too! A piezo disc might work as both transmitter and receiver, but might need more signal conditioning than an LED. Still, that's a method used things like communications between game console gimmicks [1], but also for decades in communications between devices over telephones, especially payphones¹.

What I'd really recommend, from a cost and reliability point of view: Your devices might have a plug for an external power supply. Simply add a series LC band pass filter from that to allow communication over that line! No additional connectors, and connecting a power supply to that is a testing step you're probably doing anyways. Just a filter, and some kind of an edge detector (a tinyLogic NAND gate would do) to allow for differential data transmission or a threshold (schmitt trigger, might already be part of your microcontroller).

To allow bidirectional communication over a single line, you could either go the frequency-division multiplexing route (works for both the audio as well as the powerline case), or just invent a simple "the teststand is always the initiator" protocol, where the stand sends in a sequence with a checksum or so, and then switches to reception mode for long enough for the device under test to answer. That way, you can simply use UART modules to transceive.

The easiest solution might still be the LED, and assuming you're not producing billions of your devices, I'd argue that this wins, because it needs nearly no hand-holding. In fact, I once put up a flashing LED connected to a UART in a window and told students nothing about what it does – and they still figured it out.

[1]: Vicky Pfau, "Z-Ring Phreaking from a Gameboy", in: PoC||GTFO 0x14, March 2017

¹ research the term "phreaking" if your desire to know more intensifies.

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  • \$\begingroup\$ Thank you, this was very helpful. So the idea of using near EMI for communication is not viable in your opinion? \$\endgroup\$
    – M T
    Jun 14, 2018 at 11:28
  • \$\begingroup\$ it's certainly viable, but you'll need to either have complicated signal amplification and recovery, or a significant loop to basically work as the secondary of an air-core transformer :) \$\endgroup\$ Jun 14, 2018 at 11:31
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Apologies if you have already, but have you considered using IR to communicate? It solves both the low cost and connector issue, whilst being reliable and simple to set up. Such an LED could be hidden in plain sight, behind a translucent bezel around the LCD - should it have one.

Sparkfun have a great intro to Infrared communications - it may be a bit below your level but contains helpful info anyway.

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    \$\begingroup\$ Note that this is my favourite method. In fact, you can even double-use an LED as photodiode – it's fair to assume that you could just use a vastly overpowered IR LED on your teststand site to transmit to that LED, so that the receiver in the device doesn't need to be very sensitive (making the whole system even robuster against random light fluctuations). \$\endgroup\$ Jun 13, 2018 at 15:20

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