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sorry for my bad english i have problems with rs modbus , my project is about max 485 i have master and 3 slaves when i put master with one slave it's ok and there isn't any problems but when i put the master with 2 slave or more the rs modbus is disappear ""when i invert A&B in the second slave the led of rx flashed but there is no response from processor"" what is the problem in this?? (first picture for master) (second one for slave)

the processor is atmega 8

the eagle for master

the eagle for slave

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  • \$\begingroup\$ You shouldn't terminate each node. You should have only two termination points at both ends of your bus. \$\endgroup\$ – GAttuso Dec 11 '16 at 12:01
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  1. Make sure that different slaves have different Modbus addresses.

  2. You have strong biasing resistors (470R), they should not be present on every device. Remove R16, R25, R23 and try. Most of the time the bus will work fine without them.

  3. Remove termination resistors from every but the first and last device on the bus. At low speed (eg. 9600bps) the bus will work even without them.

  4. Try multiple slaves at lower speed (eg. 9600bps). If it works then you have a signal integrity problem.

  5. Test with a known good implementation slave (eg. FreeModbus).

  6. Get a USB<->RS485 converter and test with a known good master implementation on the PC (eg. QModMaster).

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