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4000 Modbus Implementation
9.4 Functions and Data Formats
The Modbus data in the ATT is arranged in integer registers, floating point registers, and status bits. The assignments
for these registers are found at the end of this appendix.
Function codes 03, 04, 06, and 16 are used with integer registers.
Because of the multiple Modbus hosts available today, the ATT supports two floating-point data formats: a two 16-
bit register format and a one 32-bit register format. These two formats provide maximum system flexibility. In the
two 16-bit registers format, function codes 03 and 04 are used to read floating-point registers while function code
16 is used to write floating-point registers. In the one 32-bit register format, function code 65 is used to read floating-
point registers, while function code 66 is used to write floating-point registers.
Function codes 01, 02, 05, and 15 are used with status bits.
A complete description of all the preceding commands, except floating point, can be found in the Modicon Modbus
Protocol Reference Guide, document number PI-MBUS-300 Rev B.
9.5 Integer Registers
Integer registers are the most commonly used type of Modbus data and are supported by most Modbus hosts. In the
ATT implementation, the Modbus registers are arranged in one of the following four formats:
• Integer Data - a scaled number from 0 to the maximum Modbus integer
• Character Data - 2 ASCII characters per 16-bit register (ex. date, password)
• Coded Data - Multiple choice configuration data chosen from a coded list
• Packed Bit Data - Register form of 16 packed single bits
The integer, character, and coded data registers contain all of the information needed to configure and read process
data. Any integer register may be read with function code 03 or function code 04. These same registers may be written
one at a time with function code 06 or multiple registers can be written with function code 16.
For future compatibility, the ATT accepts reads and writes to reserved registers. Writes to reserved registers have no
effect. Reads from reserved registers return a zero (0).
Term Definition
Address User-assigned address of the slave device
Function Code Function the slave is to perform
Start Register (H) High-order data address byte of the number of registers to read
or write
Start Register (L) Low-order data address byte of the first register to read or write
Number of Registers (H) High-order byte of the number of registers to read or write
Number of Registers (L) Low-order byte of the number of registers to read or write
Byte Count Number of data bytes
Data MSB Data register's most significant byte
Data LSB Data register's least significant byte
Status Bit (H) High-order data address byte of the first bit to read or write
Status Bit (L) Low-order data address byte of the first bit to read or write
Error Check Message checksum CRC (Cyclical Redundancy Check)