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I want to communicate device with RS-485 bus with STM32 Nucleo-F767ZI. I have an RS-232 to RS-485 converter powered with 12V and connected to device A, B. Will it work if I connect RS-232 side TX RX to Nucleo USART or do I have to apply something to this circuit?

I'm using an RS-232 to RS-485 Sintechi:

enter image description here

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  • \$\begingroup\$ what's the output voltage levels of your converter? Also what's the converter you're using? Please include its datasheet in your question. \$\endgroup\$
    – Rahmany
    Nov 29, 2022 at 14:42
  • \$\begingroup\$ i edited post witch pins use to check output voltage lvl? \$\endgroup\$
    – username3211
    Nov 29, 2022 at 14:56
  • \$\begingroup\$ You can measure the voltage between RXD/TXD and GND \$\endgroup\$
    – Rahmany
    Nov 29, 2022 at 15:00
  • \$\begingroup\$ the output is 9volt \$\endgroup\$
    – username3211
    Nov 29, 2022 at 18:29

2 Answers 2

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You need a RS-232 interface to your Nucleo if you intend to connect to an RS-232 device (the adapter). You can't connect it directly to 3.3V logic level UART.

Of course you could modify the adapter so you can connect logic level UART directly to the MAX485 chip so you don't have any RS-232 circuitry in between.

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  • \$\begingroup\$ so if i use UART TTL RS485 MAX485 5V ARDUINO i can connect nucleo straight with rs485 device? Can this max485 still be powered with 12V beacuse i want to take power from the device with rs485 bus \$\endgroup\$
    – username3211
    Nov 29, 2022 at 18:29
  • \$\begingroup\$ I don't know what you are looking at based on some list of words that could be from a dubious online shop, which does not seem know what the sold product is or expects the buyer to not know either. But if it is a MAX485 chip then it will get damaged with 12V supply. \$\endgroup\$
    – Justme
    Nov 29, 2022 at 19:22
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Since the Output is +/- 9V, then you can't connect it directly to the MCU. As Justme said, you can use a RS-232 to TTL Uart converter.

But if you don't have a converter at hand, you can always make one with

  • a 9V-to-3.3V level shifter (with transistors) and two NOT logic gates (by an IC or transistors) because RS-232 has inverted polarity (+9V => 0V TTL; -9V => 3.3V TTL)

or

  • a rail-to-rail OPAmp comparator with a voltage divider at the input and a diode to protect against the -9V palets

If you're interested in building it, I can expand my answer.

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