max 485 ground issue

[raman00084]

i have 2 max 485 ic connected to 2 pic micro controllers one controller transmits and another receives half duplex communication, both the controllers operate from different power supply the ground is not common. my doubt is in the tx side i have one max 485 with a and b bus terminals, and rx side i have one max 485 with a and b bus terminals. if i connect tx a to rx a and tx b to rx b alone is enough are i must connect both ground together.

kindly help 2 wire are 3 wire


regards
kalyan

 

[KlausST]

Hi,

You need to connect both grounds.

Usually RS485 has a common mode input voltage range of +/- 7V, you need to ensure this, so floating is no option.

Klaus

 

[hobbyckts]

Yes, you need to connect the grounds. Otherwise the circuit will not complete a cycle and it will lead to some junk characters transfer when you cross-verify it.