I2C communication problem

[hodahussein]

dear all

i have done communication bus based on i2c between four electronic circuits
each of them include microcontroller (pic16f877) also each board has a private power supply.
so if them work and "all power supply are on " there no problem but if any power supply failure occurs the communication bus will stuck and hang up
is any microcontroller effect on bus if its own power supply become off?

what is the reason?
wait your help

 

[lucbra]

I would recommend using I2C isolators. You have ground loop problems

 

[FvM]

Unlike most dedicated I2C interface devices, PIC processors have clamp diodes between I2C GPIO pins and VDD. So they will short the I2C bus lines, if VDD is low. You would need additional bus switches to avoid this behaviour.