in fact i have several boards like Analog barrier, digital barrier, uart barrier and power control. they are connected to together by RS485. but some times when some of 485 don't work of the RX led remains on. i don't know how solve this problem. some times i have to change 485 ic for five times.

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  • \$\begingroup\$ Is the ground connection common for all boards? RS485 has a common mode voltage specification and may not work if the ground potentials are too different. \$\endgroup\$
    – Russell McMahon
    Commented Feb 1, 2021 at 11:30
  • \$\begingroup\$ yes of course all boards feed from power 48v and commen GND @RussellMcMahon \$\endgroup\$
    – kazem pch
    Commented Feb 1, 2021 at 12:14
  • 1
    \$\begingroup\$ The schematic on the bottom - is that's what's in the master, or is that in each of the other boards (Analog, Digital, UART, Power Control)? What kind of distances are involved - the horizontal run and the vertical runs in your sketch? And where is the Rx LED you mentioned? \$\endgroup\$
    – SteveSh
    Commented Feb 1, 2021 at 12:41
  • \$\begingroup\$ @kazem - Hi, This question is a repeat (with more information) of your previous question. Please do not repeat the same question. Instead you should have edited the previous question to update it - but as things have progressed here, it is too late for that now. Instead I will vote to close your old question as a duplicate of this one (and I encourage other voting members to do the same), so that no further time is wasted on that one. Please read the tour and help center to see more site rules. Thanks. \$\endgroup\$
    – SamGibson
    Commented Feb 1, 2021 at 14:23
  • \$\begingroup\$ @kazem - You replied to an earlier comment: "all boards feed from power 48v and commen GND". It is still not clear if you actually have a separate Gnd wire in your RS-485 cable, or if you are expecting that power cabling (which you have not shown or explained) is providing the common ground. IMHO we could better understand your real configuration if you added some photos of your hardware, the RS-485 cabling and overall physical layout. Also explain the environment e.g. industrial site with large motors with long RS-485 cables, or small lab with 1m RS-485 cables, or whatever. Thanks. \$\endgroup\$
    – SamGibson
    Commented Feb 1, 2021 at 14:32

2 Answers 2


RS485 needs a 120ohm terminating resistor at the end of the bus, and it is missing in your schematics.

enter image description here

(Notice my artistic talents)

  • 1
    \$\begingroup\$ Your talents are indeed good. \$\endgroup\$
    – Andy aka
    Commented Feb 1, 2021 at 13:40
  • 2
    \$\begingroup\$ At both ends of the bus. \$\endgroup\$ Commented Feb 1, 2021 at 18:12

The serial 330 ohm resistors must be replaced by 10 ohm resistors. Otherwise this will exceed the allowable line resistance without taking into account any transmission line.

  • \$\begingroup\$ Did you mean 100 ohm? \$\endgroup\$
    – WedaPashi
    Commented Feb 1, 2021 at 13:57
  • \$\begingroup\$ no 10 ohm. Check AN960 of Analog Devices where it is clearly noted: "Some TVS devices have been specifically designed for RS-485 applications. For higher power transients, protection can be increased by adding Resistors RS (between 10 Ω and 20 Ω) between the protected device and the input pin" \$\endgroup\$ Commented Feb 1, 2021 at 14:02
  • \$\begingroup\$ Ah, my bad. I thought you were referring to terminating resistance. \$\endgroup\$
    – WedaPashi
    Commented Feb 1, 2021 at 14:03

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