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Chapter 8 Communication Functions
8-57
8.3 Modbus Protocol Communication
8.3.1 Introduction
MASTER-K120S built-in communication supports Modbus, the Modicon product’s communication protocol. It supports ASCII
mode, using ASCII data and RTU mode using Hex data. Function code used in Modbus is supported by instruction and
especially function code 01, 02, 03, 04, 05, 06, 15 and 16. Refer to "Modicon Modbus Protocol Reference Guide"
8.3.2 Basic Specification
1) ASCII mode
(1) It communicates, using ASCII data.
(2) Each frame uses ': (colon : H3A)', for header, CRLF (Carriage Return-Line Feed : H0D H0A), for tail.
(3) It allows Max. 1 second interval between characters.
(4) It checks errors, using LRC.
(5) Frame structure (ASCII data)
Item Header Address Function code Data LRC Tail(CR/LF)
Size 1 byte 2 bytes 2 bytes n bytes 2 bytes 2 bytes
2) RTU mode
(1) It communicates, using hex data.
(2) There's no header and tail. It starts with address and finishes frame with CRC.
(3) It has at least 3.5 character times between two frames.
(4) It ignores the current frame when 1.5 character times elapse between characters.
(5) It checks errors, using 16 bit CRC.
(6) Frame structure (hex data).
Item Address Function code Data CRC
Size 1 byte 1 bytes n bytes 2 bytes
1
)
The size constituting 1 letter is 1 character. So 1 character is 8 bits that is 1 byte.
2
)
1 character time means the time lapsed for sending 1 character.
Ex) Calculation of 1 character time at 1200 bps.
1200 bps means that it takes 1 second to send 1200 bits. To send 1 bit, 1 sec/1200 bits = 0.83 ms.
Therefore 1 character time is 0.83ms * 8 bits = 6.64ms.
3
)
584, 984 A/B/X executes frame division, using intervals of more than 1 sec without LRC in processing internally.
REMARK