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282 • Common MODBUS functions Description of the MODBUS functions

6.2 Description of the MODBUS functions

All MODBUS functions in the WAGO ETHERNET fieldbus coupler and controller are executed as follows:

When a function code is entered, the MODBUS master (i.e. PC) makes a request to the coupler/controller of the fieldbus node.

Subsequently, the coupler/controller sends a datagram to the master as a response.

If the coupler receives an incorrect request, it sends an error datagram (Exception) to the master.

The exception code contained in the exception has the following meaning:

Exception Code

Meaning

0x01

Illegal Function

 

 

0x02

Illegal Data Address

 

 

0x03

Illegal Data Value

 

 

0x04

Slave Device Failure

 

 

The following chapters describe the datagram architecture of request, response and exception with examples for each function code.

Note

In the case of the read functions (FC1 – FC 4) the outputs can be additionally written and read back by adding an offset of 200hex (0x0200) to the MODBUS address.

Modular I/O System

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Common MODBUS functions • 283

Function code FC1 (Read Coils)

6.2.1Function code FC1 (Read Coils)

The function reads the status of the input and output bits (coils) in slave.

Request

The request determines the starting address and the number of bits to be read. Example: An inquiry, with which the bits 0 to 7 are to be read.

Byte

Field name

Example

Byte 0, 1

Transaction identifier

0x0000

 

 

 

Byte 2, 3

protocol identifier

0x0000

 

 

 

Byte 4, 5

length field

0x0006

 

 

 

Byte 6

unit identifier

0x01 not used

 

 

 

Byte 7

MODBUS function code

0x01

 

 

 

Byte 8, 9

reference number

0x0000

 

 

 

Byte 10, 11

Bit count

0x0008

 

 

 

Response

The current values of the inquired bits are packed in the data field. A 1 corresponds to the ON status and a 0 to the OFF status. The lowest value bit of the first data byte contains the first bit of the inquiry. The others follow in ascending order. If the number of inputs is not a multiple of 8, the remaining bits of the last data byte are filled with zeroes (truncated).

Byte

Field name

Example

.....

 

 

 

 

 

Byte 7

MODBUS function code

0x01

 

 

 

Byte 8

Byte count

0x01

 

 

 

Byte 9

Bit values

0x12

 

 

 

The status of the inputs 7 to 0 is shown as byte value 0x12 or binary 0001 0010.

Input 7 is the bit having the highest significance of this byte and input 0 the lowest value.

The assignment is thus made from 7 to 0 with OFF-OFF-OFF-ON-OFF-OFF-

ON-OFF.

 

 

 

 

 

 

 

 

Bit:

0

0

0

1

0

0

1

0

 

Coil:

7

6

5

4

3

2

1

0

 

Exception

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Byte

 

Field name

 

 

 

Example

.....

 

 

 

 

 

 

 

 

 

 

 

 

 

Byte 7

 

MODBUS function code

0x81

 

 

 

 

 

 

 

Byte 8

 

Exception code

 

 

 

0x01 or 0x02

 

 

 

 

 

 

 

 

 

 

Modular I/O System

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284 • Common MODBUS functions Function code FC2 (Read Discrete Inputs)

6.2.2Function code FC2 (Read Discrete Inputs)

This function reads the input bits in the slave.

Requests

The request determines the starting address and the number of bits to be read. Example: An inquiry with which the bits 0 to 7 are to be read:

Byte

Field name

Example

Byte 0, 1

Transaction identifier

0x0000

 

 

 

Byte 2, 3

protocol identifier

0x0000

 

 

 

Byte 4, 5

Length field

0x0006

 

 

 

Byte 6

unit identifier

0x01 not used

 

 

 

Byte 7

MODBUS function code

0x02

 

 

 

Byte 8, 9

reference number

0x0000

 

 

 

Byte 10, 11

Bit count

0x0008

 

 

 

Response

The current value of the inquired bit is packed into the data field. A 1 corresponds to the ON status and a 0 the OFF status. The lowest value bit of the first data byte contains the first bit of the inquiry. The others follow in an ascending order. If the number of inputs is not a multiple of 8, the remaining bits of the last data byte are filled with zeroes (truncated).

Byte

Field name

Example

.....

 

 

 

 

 

Byte 7

MODBUS function code

0x02

 

 

 

Byte 8

Byte count

0x01

 

 

 

Byte 9

Bit values

0x12

 

 

 

The status of the inputs 7 to 0 is shown as a byte value 0x12 or binary 0001 0010.

Input 7 is the bit having the highest significance of this byte and input 0 the lowest value.

The assignment is thus made from 7 to 0 with OFF-OFF-OFF-ON-OFF-OFF-

ON-OFF.

 

 

 

 

 

 

 

 

Bit:

0

0

0

1

0

0

1

0

 

Coil:

7

6

5

4

3

2

1

0

 

Exception

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Byte

 

Field name

 

 

 

Example

.....

 

 

 

 

 

 

 

 

 

 

 

 

 

Byte 7

 

MODBUS function code

0x82

 

 

 

 

 

 

 

Byte 8

 

Exception code

 

 

 

0x01 or 0x02

 

 

 

 

 

 

 

 

 

 

Modular I/O System

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Common MODBUS functions • 285

Function code FC3 (Read multiple registers)

6.2.3Function code FC3 (Read multiple registers)

The binary contents of holding registers are read from the slave using this function.

Request

The request determines the start word address (start register) and the number the register to be read. The addressing starts with 0.

Example: An inquiry of the registers 0 and 1:

Byte

Field name

Example

Byte 0, 1

Transaction identifier

0x0000

 

 

 

Byte 2, 3

protocol identifier

0x0000

 

 

 

Byte 4, 5

length field

0x0006

 

 

 

Byte 6

unit identifier

0x01 not used

 

 

 

Byte 7

MODBUS function code

0x03

 

 

 

Byte 8, 9

reference number

0x0000

 

 

 

Byte 10, 11

Word count

0x0002

 

 

 

Response

The reply register data is packed as 2 bytes per register. The first byte contains the higher value bits, the second the lower values.

Byte

Field name

Example

.....

 

 

 

 

 

Byte 7

MODBUS function code

0x03

 

 

 

Byte 8

Byte count

0x04

 

 

 

Byte 9, 10

Value Register 0

0x1234

 

 

 

Byte 11, 12

Value Register 1

0x2345

 

 

 

The contents of register 0 are displayed by the value 0x1234 and the contents of register 1 is 0x2345.

Exception

 

 

 

 

 

Byte

Field name

Example

.....

 

 

 

 

 

Byte 7

MODBUS function code

0x83

 

 

 

Byte 8

Exception code

0x01 or 0x02

 

 

 

Modular I/O System

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286 • Common MODBUS functions Function code FC4 (Read input registers)

6.2.4Function code FC4 (Read input registers)

This function serves to read a number of input words (also ”input register”).

Request

The request determines the address of the start word (start register) and the quantity of the registers to be read. Addressing starts with 0.

Example: An inquiry of the registers 0 and 1:

Byte

Field name

Example

Byte 0, 1

Transaction identifier

0x0000

 

 

 

Byte 2, 3

protocol identifier

0x0000

 

 

 

Byte 4, 5

length field

0x0006

 

 

 

Byte 6

unit identifier

0x01 not used

 

 

 

Byte 7

MODBUS function code

0x04

 

 

 

Byte 8, 9

reference number

0x0000

 

 

 

Byte 10, 11

Word count

0x0002

 

 

 

Response

The register data of the answer is packed as 2 bytes per register. The first byte has the higher value bits, the second the lower values.

Byte

Field name

Example

.....

 

 

 

 

 

Byte 7

MODBUS function code

0x04

 

 

 

Byte 8

Byte count

0x04

 

 

 

Byte 9, 10

Value Register 0

0x1234

 

 

 

Byte 11, 12

Value Register 1

0x2345

 

 

 

The contents of register 0 are shown by the value 0x1234 and the contents of register 1 is 0x2345.

Exception

 

 

 

 

 

Byte

Field name

Example

.....

 

 

 

 

 

Byte 7

MODBUS function code

0x84

 

 

 

Byte 8

Exception code

0x01 or 0x02

 

 

 

Modular I/O System

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