I followed this guide to get an additional serial port set up on my Nano 33 IoT, and it worked like a charm. The only piece I'm missing now is: how do I invert this UART? I 'm trying to talk to my water heater using the GEA2 protocol, which is an inverted serial protocol.
Here is another topic discussing this very question for an Itsy Bitsy, which uses a different variant of the SAMD (SAMD51), but I'm not 100% sure how to translate this over to the SAMD21 in the Nano 33 IoT.
@MartinL I wonder if you might have an idea of where I might get started on this? On a previous post, you shared some excellent info on how to achieve this on an Itsy Bitsy, but I'm having some trouble achieving the same on my board. Any insight you can provide is greatly appreciated!
I looked up the GEA2 protocol, which is an encrypted protocol. I can't see any reference to an inverted protocol in this link.
Serial protocols like RS232 use a convention that logic 0 is sent as high and logic 1 is sent as low. However these signals also have the voltage totally different from a normal TTL signal, with high (representing a logic 0) being +12V and low being -12V. So you have to convert the voltages into TTL (0 - 5V) and then do the logic inversion.
This can not be done in software alone, but needs some hardware to do the conversion. A chip like a MAX232. There are a lot of alternatives, and you can even buy ready made boards that do this for not very much money on sites like eBay.
Ah, I had come across this previously. There are 2 protocols who have decided to call themselves GEA2, haha. I'm referring to the one used by General Electric appliances:
You will need to do the inversion in hardware.
I found this schematis of a GE appliance adaptor board.
Schematic1.pdf (58.4 KB)
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