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I am currently trying to communicate with a modbus slave through a RS485/Ethernet converter. The slave device is RS485 2 wire, and the converter is RS485 4 wire. As of now, I wired both Tx+ and Rx+ to the slave's Data+ pin and the same for its data- pin. However, when communicating I only get echoes of the master's queries instead of proper answers from the slave. According to this forum the converter I use might not shut down its receivers when transmitting, thus the echoes I get.

After looking around I found two possible ways to fix the problem:

  • use a converter: either buy it or make it, although I haven't found any projects about this online so maybe making it isn't worth it

  • use something similar to this isolator for I2C, which prevent signals from coming back to their origin

Are those the only possibilities I've got? I feel like buying a converter will be the final solution but I also wanted to know about the ways to go around this problem.

Here's a sketch for more details:

wiring sketch

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  • \$\begingroup\$ "The slave device is RS485 2 wire" ... "As of now, I wired both Tx+ and Rx+ to the slave's Data+ pin" How does this make any sense? Does it have 1 differential signal or 2 differential signals? Or does it have no differential signal at all...? \$\endgroup\$
    – Lundin
    Aug 11, 2021 at 9:33
  • \$\begingroup\$ you have a bad modbus master implementation. \$\endgroup\$ Aug 11, 2021 at 10:05
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    \$\begingroup\$ A schematic or sketch is always helpful. Please provide something along that line. \$\endgroup\$
    – SteveSh
    Aug 11, 2021 at 10:07
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    \$\begingroup\$ More importantly, the 4-wire device is not shutting down its transmitter when it should be listening for the device response. See electronics.stackexchange.com/a/69898/11683 \$\endgroup\$
    – Dave Tweed
    Aug 11, 2021 at 10:38
  • \$\begingroup\$ @MarkoBuršič I am using pyModbus for my master implementation, could you elaborate on how it is bad exactly ? \$\endgroup\$ Aug 11, 2021 at 14:01

2 Answers 2

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From EDN we have this circuit diagram. Now quite often RS485 works without pull up/down resistors, but not always

enter image description here

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  • \$\begingroup\$ Anuja Mehendale just caught this pulllup/down problem with a 485 connected device that did not work without them. All other variants of that type of device don't need them \$\endgroup\$ Aug 11, 2021 at 14:06
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As Marko Buršič implied, the problem was coming from the RS485/EtherNet converter. After looking deeper into the product I discovered it was not capable of being a Modbus gateway. I was using ICP DAS' tDS-715, by the way.

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