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This is the first time in my life I need to try "Modbus RTU", and as a master I need a RS485 to USB converter which will establish connection to the PC.

I have found some converters in the market which stress and mention their product is Modbus compatible such as this one: KMP Electronics Convertor from USB to RS485 protocol GALVANICALLY ISOLATED MODBUS compatible.

On the other hand there are many other RS485 to USB converters which does not mention Modbus at all.

Below is my basic plan:

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I found a converter for this purpose, but its data-sheet does not include a word about Modbus. Isn't Modbus a protocol and the converter itself a physical layer? What is being meant here by saying "Modbus compatible"? Most importantly, how can I know whether this converter: USB RS485 Cable Series - FTDI supports Modbus RTU or not?

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  • \$\begingroup\$ FYI: Links eventually go dead or the content changes. Be sure to put something in your text that gives an idea of what you're linking to. In this case, replace "this" with a model number. \$\endgroup\$
    – Bort
    Apr 18, 2018 at 18:41

2 Answers 2

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Modbus is a protocol that typically uses RS485 physical interface. Any adapter that is RS485 compliant should work with Modbus.

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  • \$\begingroup\$ So writing extra "Modbus compatible" to an RS485 to USB converter seems to be a marketing trick then? \$\endgroup\$
    – user1999
    Apr 18, 2018 at 19:02
  • \$\begingroup\$ They probably know that is the largest market for their product. \$\endgroup\$
    – EE_socal
    Apr 18, 2018 at 19:08
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Yes, any viable RS-485 adapter will work with the Modbus protocol (no matter if Modbus-RTU or Modbus-ASCII is required). Actually, the RS-485 interface is used for other protocols as well (Profibus-DP, DMX, ...).

I'd recommend to only buy converters that offer at least 3 terminals for A+, B- and GND. Some very cheap converters don't offer the GND terminal which may cause problems IMO.

If the cabling is long, or unshielded, or crosses other power lines, or the connected devices are power separately etc. a galvanic/optical isolation can be beneficial as well. It will cost you some extra bucks but it is worth it. Search for "485 isolated" on Amazon.

I have a bunch of DSD TECH SH-U11H which made me happy, but also the Waveshare parts may be fine. There are masses of other brands available.

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