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RS485 connection with microcontroller

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hemnath

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Hi,
I have connected ISL83485 RS485 transceiver to microcontroller. Attached pdf for your reference. I have to design a pcb where the customer/user will use RS485 communication.

Should I open the connection on A and B where the user will connect the 120Ohms resistor (or) should i connect 120 Ohms permanently. Any Protection components?
 

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  • Sheet1.pdf
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Put 120 Ohm resistor on your PCB and a jumper for possibility to terminate the line.
If your PCB is first in the communication chain, termination resistor is mandatory, as well as at the last transciever on the line.
 

The resistor is part of the line termination so it should be there unless one is being provided externally. It is common for a plug-in link to be used so the end user has the option to use it or not.

Protection isn't normally needed but if you want to add it, make sure there is a ground connection between the devices on the lines and that the voltages on the lines stay within the limits allowed by the ISL83435 and its ground/supply pin. I do not have its data sheet to hand to see what the limits actually are.

Brian.
 

Thanks. I'll add 120 Ohms resistor. When i checked for protection, i came across the attached circuit. Why 10 Ohms are used. Is this basic level of protection is normally enough for RS485 connection?.
with protection.jpg
 

Normally it isn't needed at all but there is no harm in adding it. The 10 Ohm resistors are just to limit the current flowing to/from D2 if one of the data lines is taken outside the safe range. Without then D2 would probably be killed but note the 1W rating to allow the resistors to survive a bit longer under overvoltage conditions. Values higher than 10 Ohms might limit the distance or number of slaves that can be driven. Consider that there should be another 'R8' at the far end of the D+/D- wires so any series resistance will drop the available voltage to some extent.

Brian.
 

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    hemnath

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Perhaps reviewing this document will be useful for you. The position of the TVS diodes is very different.
 

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