Varec 2920, Инструкция по установке и эксплуатации, Страница: 161 / 194

MODBUSAppendix A — MODBUS Implementation
Varec, Inc. 151
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.
Integer Registers
Integer registers are the most commonly used type of MODBUS data and are supported by
most MODBUS hosts. In the 2920 FTT 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
• Coded Data - Multiple choice configuration data chosen from a coded list
• Packed Bit Data - Register form of 16 packed single bits
The integer, 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 2920 FTT accepts reads and writes to reserved registers. Writes to
reserved registers have no effect. Reads from reserved registers will return undocumented
values.
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)