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I am trying to configure a Pi 4 and eventually a CM3 for modbus communication. so far I have been able to see the TX send from the D+ and D- of the max485cpa+ but I have been unable to receive data via RX (via rs485 or rs232).

Today I set a meter on the RX pin and found that the pin was high, and I know that the Pi wont receive if RX is high.

Is this a defect of the board or is there a way to set RX low to receive data?

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  • It's high because it's set as an input with a pull-up - as long as gpioinfo shows it as being an input, it will work as an input – Bravo Jul 21 at 23:18
  • It is set as an input but will not receive data. I’ve tested this in a number of different ways but the results are the same. My understanding is that with uart enabled the rx pin floats and when tested should read zero volts. Is this incorrect? – pmichaels Jul 22 at 2:17
  • if it's set as an input it CAN receive data - perhaps you're sending device isn't sending data, perhaps your connections are incorrect, perhaps you haven't configured the pi correctly - no idea, but the fact that RX is high is NOT the problem – Bravo Jul 22 at 2:19

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