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The schematics under question is a 3-phase metering board. It uses a number of isolators. The main MCU of the board is STM8S003K3. Its RX/TX pins are connected to the input an isolator (ADUM1412) and the output of the isolator is connected to FT232BL and through some transistors to an RS485 chip (ST485B). I understand the connection between ADUM1412 and FT232BL, what I don't get is how RS485 fits in the picture. Image of the RS485 section

Image of the Isolator(ADUM1412) section

Image of the USB to UART section (FT232BL)

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  • \$\begingroup\$ What specifically is it that you don't understand. Seems clear enough to me. \$\endgroup\$
    – Andy aka
    Apr 19, 2020 at 16:25
  • \$\begingroup\$ I don't understand the communication between the (MCU and RS485) and (USB and RS485) \$\endgroup\$
    – Rockson
    Apr 19, 2020 at 16:36

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STM8 RX pin can receive data either from USB via Q1 or RS485 via Q4, will both affect the STM8 RX pin via the isolator.

STM8 TX pin can transmit data via the isolator, it goes directly to USB RX pin and via Q2 to RS485 transmit enable pin.

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  • \$\begingroup\$ I get the first part now. Considering transmission via isolator and via Q2 to RS485, How does transmission through RS485 work when its Driver Input (DI) is tied to ground? \$\endgroup\$
    – Rockson
    Apr 19, 2020 at 23:29
  • \$\begingroup\$ By using the the transmit enable pin, to either send logic 0, or not sending anything which is logic 1. \$\endgroup\$
    – Justme
    Apr 19, 2020 at 23:33
  • \$\begingroup\$ Yeah. Not sending anything is logic 1 because the non-inverting output is is pulled up. Thank you so much. \$\endgroup\$
    – Rockson
    Apr 20, 2020 at 0:06

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