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Zeshan

Communication

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Hello.

I am having 22 slaves communicating to a master on rs485 half duplex.i have selected baud rate as 38400 on a length of 800m.The problem is out of 22 devices only six are communicating .i have checked wiring,inserted termination resistor but still not getting rest of the devices alive.is it that i have selected high baud rate on a long length?this is the only problem as i am seeing it.can somebody help me on this issue

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From what I could google that's twice as fast as you should run at that distance.

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I have modicon m580 cpu as master and tm221c16r as slaves and etor4 as gateway.but i cant get all devices in communication

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Yes, the baud rate is excessively high. What cable are you using? I am assuming that you have no duplicate addresses. What is the terminating resister value? What is the resistance across the network cable?

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The value of terminating resistors is 120 ohms and i am using beldan 3 shielded cable.really dont know the line resistance.should i try less baud rate as 9600?or 192000?plz help me

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speed vs. distance depends on used communication chips. not all are created equal, some have lower output impedance than others and can drive higher current. many fieldbusses are based of 485, and they have different specs regarding cable and terminating resistors.

terminations must match the cable. some networks use 82 Ohm, 120 Ohm, etc... or series if 1kOhm and 1nF capacitor. picking some random cable (and Belden makes MANY kinds) and random termination is not a good way to start a project. 

i am sure your product manuals have details on that. in general lower baud rate is less sensitive since slower operating time allows signal artefacts (ringing, edge rounding etc). to settle down. this is why some network support wide range of baud rates.  if this is a suspected problem, simply pick the slowest rate supported (1200?) and work your way up.

but also check if you have duplicate nodes, ground loops, routing near interference sources etc. there could be number of other reasons this is not working as expected.

 

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