RS485 biasing for Modbus

Modbus.

Try this perhaps...

Modbus.

Try this...

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:

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

Marc

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: