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Appendix A: Reference Guides 121 (124)
A Reference Guides
A.1 About Input Registers and Holding Registers
Modbus data is most often read and written as registers which are 16-bit pieces of data.
Holding registers and Input registers are both 16-bit registers.
Input registers
Input registers can only be read.
Holding registers
Holding registers can be read or written.
These registers can be used for a variety of things such as inputs, outputs, configuration data, or
other requirement for holding data.
A.2 Modbus Data Model
Discretes Input Single bit Read-Only
Data can be provided by the I/O system.
Coils Single bit Read-Write Data can be alterable by the application program.
Input Registers 16-bit word Read-Only
Data can be provided by the I/O system
Holding Registers 16-bit word Read-Write Data can be alterable by the application program.
Reference: MODBUS Application Protocol Specification V1.1b3, April 26 2012
For more information refer to the Modbus organisation website.
A.3 Modbus Transactions
Nr
Transaction Function Code Description
1 Read Coils 0x01 Read from 1 to 2000 contiguous status of coils in a remote device.
2 Read Discrete Inputs 0x02 Read from 1 to 2000 contiguous status of discrete inputs in a remote
device.
3 Read Holding Registers 0x03 Read the contents of a contiguous block of holding registers in a
remote device.
4
Read Input Registers 0x04 Read from 1 to 125 contiguous input registers in a remote device.
5
Write Single Coil
0x05
Write a single output to ON or OFF in a remote device.
6 Write Single Register 0x06 Write a single holding register in a remote device.
15 Write Multiple Coils 0x0F In a sequence of coils, force each coil to either ON or OFF in a
remote device.
16 Write Multiple
Registers 0x10 Write a block of contiguous registers in a remote device.
22 Mask Write Register 0x16 In a single transaction, modify the contents of a specified holding
register using a combination of an AND mask, an OR mask, and the
register's current contents.
Can be used to set or clear individual bits in the register.
23
Read/Write Multiple
Registers
0x17
Performs a combination of one read operation and one write
operation.
The write operation is performed before the read.
Reference: MODBUS Application Protocol Specification V1.1b3, April 26 2012
For more information refer to the Modbus organisation website.
Anybus
®
Communicator
™
User Manual
SCM-1202-152 1.10 en-US