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Unterminated RS485 bus

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jasensio1

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Hello guys.

To improve power consumption on a battery powered system, I am planning to remove the termination resistor on a RS485 bus.

It will carry point to point communications (just two transceivers attached to the bus) at low baud rate and through a short cable (2 meters long).

I am using LTC2852 transceiver but I am not sure if its failsafe feature will ensure a receiver known state when the bus is not driven and unterminated. The datasheet does not say anything about internal bias resistors so...

Should I add biasing resistors or it is not needed in this case?

Thanks
 

Most industry standard transceivers don't have an assured output level with open inputs, some like MAX485 however have so-called fail safe behavior. For all others, you should place bias resistors, otherwise the receiver will have serious difficulties. Without parallel termination, the bias resistors can be quite high, e.g. 10 k. Problem is that they won't suffice if you later decide to add parallel termination, neither the inbuilt "fail safe" termination will work in this case.
 
Hi,

For reliable data operation I recommend to use proper termination.

I've seen some circuits with a capacitor in series with the termination resistor, it will avoid reflections, but the voltage levels are distirted somehow. Therefore I can not recommend it.

Klaus
 

Everyone is running 115k RS-232 over 5 m without termination, why should RS-485 need with similar speed and cable length? Bias resistors are a different thing, they are required to enforce correct idle state if no peer is driving the line.
 

Hi,

Sorry, I didn't realize that the OP mentiones short length (2m).
Now I fully agree with FvM.

Klaus
 

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