Computer Interfacing
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Standard Modbus Serial Line Parity Setting


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Author Message

Jun 09, 2010 3:35 pm

Hello Lammert,

It appears that the reason for the Modbus standard for RTU serial line transmissions requiring that 2 stop bits be used when no parity is used was to keep the total number of bits per byte to 11 bits.

Do you know why there was a perceived need to maintain the 11 bits per byte?

The only reason for it that I can think of is perhaps some UARTs of the late 1970's couldn't handle 8N1 (i.e. no parity and 1 stop bit). However, this is often a source of confusion since today, 8N1 the de facto standard. Why shouldn't the Modbus standard allow for using 8N1?

Thanks in advance,

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