How 8051 serial mode 3 can be used in 6 devices communication ?

 

Two things:
The character frame is 11 bits long, including a START bit, 9 databits, and a STOP bit.
There is special hardware to detect when a byte arrives with the most significant bit equal to one.

Have you read the datsheet?

 

Two things:
The character frame is 11 bits long, including a START bit, 9 databits, and a STOP bit.
There is special hardware to detect when a byte arrives with the most significant bit equal to one.

Have you read the datsheet?

Yes.

Where do the 9th data bit goes to ?

 

The 9th bits for transmit and receive go to Special Function Register SCON(0x98), bits 2 & 3. They are called TB8 and RB8. Where did you think they went? Where else could they go?

 

Definitely need to check out the datasheet.
It is all in there, just take the time to go through it.

I was doing this just the other week.

 

The 9th bits for transmit and receive go to Special Function Register SCON(0x98), bits 2 & 3. They are called TB8 and RB8. Where did you think they went? Where else could they go?

Ok.
Then, how to set the baud rates of the devices (Timer values) ?

 

It depends on the 8051 variant we are talking about. My usual point of reference is the Atmel line of second source parts.
http://www.atmel.com/images/doc4316.pdf
On page 2-97 is the baudrate selection diagram. You can use Timer1, Timer2, or the internal baudrate generator.

IIRC the original 8051 from Intel had a more restricted set of choices. There was no Timer2 and no internal baudrate generator. It was common to select odd crystal frequencies so that baudrates would come out correctly with only minor errors.

 

... It was common to select odd crystal frequencies so that baudrates would come out correctly with only minor errors.

Good point, I normally only use crystals working at 22.1184 MHz, 14.7456 MHz, 11.0592 MHz and 9.8304 MHz, those values will give you exact common baudrates within reasonable speed ranges in most 8051 MCUs