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176 HVAC Drive H300
Issue Number: 3
10.3.3 MODBUS registers
The MODBUS register address range is 16 bit (65536 registers) which at the protocol level is represented by indexes 0 through 65535.
PLC registers
Modicon PLCs typically define 4 register 'files' each containing 65536 registers. Traditionally, the registers are referenced 1 through 65536 rather than
0 through 65535. The register address is therefore decremented on the master device before passing to the protocol.
The register file type code is NOT transmitted by MODBUS and all register files can be considered to map onto a single register address space.
However, specific function codes are defined in MODBUS to support access to the "coil" registers. All standard CT drive parameters are mapped to
register file '4' and the coil function codes are not required.
CT parameter mapping
The Modbus register address is 16 bits in size, of which the upper two bits are used for data type selection leaving 14 bits to represent the parameter
address, taking into account the slave increments the address value by 1, this results in a theoretical maximum parameter address of Pr 163.84
(limited to Pr 162.99 in software) when the default standard addressing mode (see Serial Mode Pr 11.024) is used.
To access a parameter number above 99 in any drive menu then the modified addressing mode must be used (see Serial Mode Pr 11.024), this will
allow access to parameter numbers up to 255 but also limit the maximum menu number to 63.
The Modbus slave device increments the register address by 1 before processing the command, this effectively prevents access to parameter
Pr 00.000 in the drive or option module.
The tables below shows how the start register address is calculated for both addressing modes.
Data types
The MODBUS protocol specification defines registers as 16 bit signed integers. All CT devices support this data size. Refer to the section
10.3.7 Extended data types on page 179 for detail on accessing 32 bit register data.
10.3.4 Data consistency
All CT devices support a minimum data consistency of one parameter (16 bit or 32 bit data). Some devices support consistency for a complete
multiple register transaction.
10.3.5 Data encoding
MODBUS RTU uses a 'big-endian' representation for addresses and data items (except the CRC, which is 'little-endian'). This means that when a
numerical quantity larger than a single byte is transmitted, the MOST significant byte is sent first. So for example
16 - bits 0x1234 would be sent as 0x12, 0x34
32 - bits 0x12345678 would be sent as 0x12, 0x34, 0x56, 0x78
10.3.6 Function codes
The function code determines the context and format of the message data. Bit 7 of the function code is used in the slave response to indicate an
exception.
File type Description Supported
1 Read only bits ("coil") Use register
2 Read / write bits ("coil") Use register
3 Read only 16bit register Yes
4 Read / write 16bit register Yes
Parameter Addressing mode Protocol register
0.mm.ppp
Standard mm x 100 + ppp - 1
Modified mm x 256 + ppp - 1
Examples
16-bit 32-bit
Decimal Hex Decimal Hex
0.01.021
Standard 120 0x00 78 16504 0x40 78
Modified 276 0x01 14 16660 0x41 14
0.01.000
Standard 99 0x00 63 16483 0x40 63
Modified 255 0x00 FF 16639 0x40 FF
0.03.161
Standard N/A N/A N/A N/A
Modified 928 0x03 A0 17312 0x43 A0
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