User`s manual

FX3U Series Programmable Controllers

User’s Manual - MODBUS Serial Communication Edition

7 MODBUS Standard Commands

7.2 Frame Specifications

b) ASCII mode

In this mode, frames are received or sent in units of 2 characters 2 bytes of ASCII codes.

The frame specifications are compliant with the MODBUS protocol specifications.

Note

The Longitudinal Redundancy Checking (LRC) field is one byte, containing an 8-bit binary value. The LRC

value is calculated by the transmitting device, which appends the LRC to the message. The device that

receives recalculates an LRC during receipt of the message, and compares the calculated value to the actual

value it received in the LRC field. If the two values are not equal, an error results.

A procedure for generating an LRC is:

1) Add all bytes in the message, excluding the starting ‘colon’ and ending CRLF. Add them into an 8-bit field,

so that carries will be discarded.

2) Subtract the final field value from FF hex (all 1’s), to produce the ones-complement.

3) Add 1 to produce the twos-complement.

4) For a transmit frame the LRC is calculated before converting to ASCII.

Start END

Error check calculation range

Address field

(3A

)

2 bytes

Function code

2 bytes

Data

Error check

n x 2 bytes

(n = 0 to 252)

2 bytes

CR + LF

(0D

) (0A

)