Modbus RTU Dataframe- 11 bits or 10 bits

[arup]

Dear sir,
I would like to know .... If i have to implement modbus RTU protocol then is it mandatory that dataframe should be 11 bits?

If i set Parity : NONE then do i require to set STOP bits : 2 ?? is it mandatory or optional.Because I have seen catalogs of few well known company who have describe their communication setting as follows :

Protocol: MODBUS RTU
Physical Link: RS485 half duplex
Comms. Mode:
Data Bit: 8
Stop bit: 1
Parity: none

Pls comment

Regards,
tech..

 

[grieblm]

According to the Modbus standard it is required to always have 11 bits, therefore you will need 2 stop bits when there is no parity bit.

Practically-speaking, I doubt that it makes a difference as long as the characters have at least one bit time between them. Most UARTs will interpret idle time as stop bits.