User`s manual

6-9

6.5 MODBUS Communication Protocol

INP-TN5A0202-E

Message composition

Command messages are composed of four parts; a station number, a function code, a data part, and an error check code, and

sent them in the order above.

Field Name Number of Bytes

Station No. 1 byte

Function code 1 byte

Data part 2 to 64 bytes

Error check code (CRC-16) 2 bytes

Station No.

This is the number allocated to a slave. It is ﬁ xed to “128”.

Function code

This code speciﬁ es the function for the slave to perform.

Data part

These data is required to run the function code. The composition of the data part varies according to function codes. Coil number

or register number is allocated to the data in the temperature controller, and specify those numbers to read out/write through

communications.

The coil number or the register number in messages uses relative addresses.

The relative address is calculated using the formula below.

Relative address = (The last 4 digits of a coil No. / register No.) -1

Ex.) When a function code speciﬁ es a register number “40003”

Relative address = (The last 4 digits of 40003) – 1

= 0002

is used in the message.

Error check code

This code is for detecting the message error (changes in the bits) during the signal transmission process. MODBUS protocol

mode (RTU mode) uses CRC-16 (Cyclic Redundancy Check).