I am designing a 300m two wire Modbus RTU network with a PLC master. I will try to daisy chain my slave devices (29 devices) but require several taps off my trunk line. How long can these taps be? I have read information saying they must be less than 1m and I have also read that the taps cannot be more than 20m in length. Please confirm.

Also..does having passive taps reduce the overall trunk length?

Thanks
Shawn

 

RS485 2 wire 1/2 duplex cannot have any "Tap length". There are only 2 ends to an RS485 highway.

 

If baud rate is below 19200 and bus lenght below 100 meters, there will be no problem with your topology. Problems arise when both (or any) of baud rate and/or bus length increases.