Dear Erik,
The system measures the weight, height( by ultrasound), body fat percentage, heart beat and blood pressure of a person and instructs on his/her health conditions considering some parameters( such as: age, man/woman, body structure(large, medium, small) and prints the results and status etc. for him/her.

The measurements are done with individual small systems as slaves, and a master controls all of them , entering the user data, and printing etc.

In order to reduce the hardware, I decided to use I2C bus which is available on all micros now instead of RS485 for example and to ease the implemetation for user I considered a PLUG and PLAY feature which frees him from knowing anything about the harware at all.

Since it requires more complex code than traditional RS232, I faced that problem, because I don't have any previous experice on it.

Dear andrew,
A CAN solution ,although I know nothing about it, needs extra hardware or more expensive micros, which hich increases cost, I suppose.

Thanks for your suggestions, I must try them all.
Ali