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I'm currently out of ideas, i have a simple Wigoneer RS485 TTL interface (Sch link here) with MAX485 wired to my ESP32 based board, in my lab everything works like a charm with 1300 meters of CAT6 UTP Cable, but when i try to put in production with 800 meters of CAT6/FTP cable nothing works (using ModbusMaster library i get 224 error)

keep in mind that the cable it's placed in a cable duct with three phase lines, so it's a totally different condition instead my lab and it's placed in a harsh environment with motors, solder machines and so on...

Anyway, i've decided to redesign my RS485 breakout board using an isolated transciever, in this case the ADM2483: it's a good idea? maybe can help me to solve this problems that i'm encountering in harsh environments?

This is the schematic reference that i'm planning to use:

VDD1: 3.3V - VDD2: 5V

ADM2483_schematic

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  • \$\begingroup\$ Do the devices share a common ground reference wire? \$\endgroup\$
    – Justme
    May 22, 2020 at 17:40
  • \$\begingroup\$ If you mean the Tranciever and the field PLC no, they only have two wires (A-B), i've tried a simple USB-RS485WE interface to test the field PLC in order to understand what was going wrong and nothing works, when i've switched to this interface (isolated: qeed.it/wp-content/uploads/2020/01/MI000180_Q-USB-ITA_ENG.pdf) magically everything started working...why??? \$\endgroup\$
    – VirtApp
    May 22, 2020 at 18:16
  • \$\begingroup\$ Is your CAT6 shielded? Or is it just basic spool stuff you buy at hardware stores? \$\endgroup\$
    – Ron Beyer
    May 22, 2020 at 18:17
  • \$\begingroup\$ It's a Commscope shielded cable certified CAT6A with external foil shield and additional pairs shield \$\endgroup\$
    – VirtApp
    May 22, 2020 at 18:18
  • \$\begingroup\$ Really, no common ground reference wire? Is the ESP32 grounded? Does it have grounded power supply, or floating? Is the device at the other end of the 800m cable also grounded, or is it floating? \$\endgroup\$
    – Justme
    May 22, 2020 at 18:35

1 Answer 1

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So as one device (PLC) has reference to mains earth, and the other device is floating (PoE by definition is isolated), they have no common ground reference, so simply two data wires of RS-485 will not work and can even break RS-485 tranceivers with large potential differences between the devices.

Standard RS-485 tranceivers such as the MAX485 allow up to 7V difference between their grounds, so this will be exceeded if the PoE device is floating. The isolated tranceiver works because it allows the isolated side ground reference to float and be set via the data wires, and it will allow higher difference between isolated ground and PC ground. Although that is not even necessary, because the PC is also most likely earthed, so there will be a common ground reference between PLC and PC, via mains earthing.

Basically, since the ESP32 is powered via PoE, you need the reference ground wire between ESP32 and PLC.

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  • \$\begingroup\$ "Basically, since the ESP32 is powered via PoE, you need the reference ground wire between ESP32 and PLC." This assumption it's valid only for not ISO transceivers, right? \$\endgroup\$
    – VirtApp
    May 22, 2020 at 22:08
  • \$\begingroup\$ "The isolated tranceiver works because it allows the isolated side ground reference to float and be set via the data wires, and it will allow higher difference between isolated ground and PC ground." How this is possible? This concept isn't very clear for me, you can do more in depth explanation? Many, many thanks man! :) \$\endgroup\$
    – VirtApp
    May 22, 2020 at 22:13
  • \$\begingroup\$ The assumption is valid for all tranceivers, isolated or not. The tranceiver has to have a reference potential with the other tranceiver that is within 7 volts. The isolated tranceiver isolates, so for example, there can be hundreds of volts between PC ground side and isolated tranceiver ground. The isolated ground must be connected to the remote traceiver ground, so that tranceivers have grounds within 7V. \$\endgroup\$
    – Justme
    May 22, 2020 at 22:27
  • \$\begingroup\$ And why using a USB rs485 isolated tranceiver everything works? I mean, my notebook is not sharing the same ground and in addition it's on battery... This is not definitely clear to me, maybe I'm missing something. \$\endgroup\$
    – VirtApp
    May 22, 2020 at 23:02
  • \$\begingroup\$ Good point, I assumed a grounded desktop PC. However, the laptop potential, grounded or not, makes no difference, because the RS-485 tranceiver isolates the computer from the bus. In either case, the isolated side common mode potential is set by data wires. \$\endgroup\$
    – Justme
    May 23, 2020 at 6:56

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