Design decision for modbus integration.

[MichaelG]

Common enough

Usually it is designed out by only having one communications link
ie Master direct to the Xbee slave
When there are multiple smart hops to the final slave typically each hop uses different addresses and the final slave is smart enough to receive commands on one register and send status from another

Another method is where each message has a unique ID that assists in knowing what is happening on the communications at both ends

 

[bluffit]

Thanks Michael

Zigbee doesn't support Modbus and adding an extra protocol layer over the serial line is probably overkill. Using two sets of registers - one to read and one to write - will work completely fine. These guys use just the one set - Zigsense.com.au - so have overcome the issue somehow.