RS485 biasing for Modbus

I am a little confused by the biasing requirements of RS485 bus for Modbus.

I normally bias the RS485 driver with A > B to ensure a HIGH at any receiver on the bus when now transmission is in progress.

The standard though states that B must be higher than A for idle state of the RS485 bus. If I bias like this, the micro in the device will continually think a start bit is being received due to a low on the RS485 IC (MAX485).

All practical applications I have seen so far bias A > B.

Could someone shed some light on this for me please.

Regards

David

Reply to
David
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Modbus.

Try this perhaps...

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Reply to
CWatters

Modbus.

Try this...

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Reply to
CWatters

Thanks for the link.

It seems I was not dreaming afterall and that there is confusion over the biasing.

So far my systems Bias A higher than B to be compatible with the microcontroller UART and not require inversion between the MX485 and Micro.

CWatters wrote:

Reply to
David

This application note does not address Modbus, but is a very good document on how to bias RS-485:

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Marc

Reply to
Marc Guardiani

Marc,

Thanks for the link. I have the RS485 bus working fine for our own products. (Using 680R pull up and pull down resistors and 120R termination). The problem is that we are now producing Modbus slave and Master devices and need to clarify the Modbus requirement for A to be biased lower than B.

If another manufacturers device is a Master and they bias A to be biased lower than B but our system is designed for bias with A more positive than B then we have an issue. I suppose if we state the requirements in the documentation the users can connect A of their master to B of our slave and visa versa.

I imagine many applications (like ours) connects the RS485 interface IC directly to a micro controller. If this is the case, they need RX to idle "1" to prevent the micro UART interrupt continually being triggered.

I suppose another way to do it is to avoid using A and B designations and simply mark the RS485 as (+) (-) and (Com).

Marc Guardiani wrote:

Reply to
David

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