Vacon CX Series - 3.2 Modbus Frame Structure; 3.3 Supported Modbus Functions; 3.4 Modbus Error Detection

VACON CX Modbus user’s manual Page 6
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3.2 Modbus frames
Two modes of transmission are available for use in a MODBUS system. The modes are ASCII (American
Standard Code for Information Interchange), and RTU, (Remote Terminal Unit.). The Fieldbus board uses
only RTU mode.
The format for each byte in RTU mode:
Coding system: 8-bit binary, hexadecimal 0-9, A-F
Two hexadecimal characters contained in each 8-bit field of the message.
Bits per Byte: 1 start bit
8 data bits, least significant bit sent first
1 bit for even/odd parity, no bit for no parity
1 stop bit if parity is used; 2 bits if no parity
Error Check Field: Cyclical Redundancy Check (CRC)
In RTU mode, messages start and end with a silent 3.5 characters times (T1-T2-T3-T4). The entire
message frame must be transmitted as a continuous stream. A typical message frame is shown below.
Start Address Function Data CRC Check End
T1-T2-T3-T4 8 bits 8 bits n*8bits 16 bits T1-T2-T3-T4
The individual slave devices are assigned addresses in the range of 1 - 247. Address 0 is used for the
broadcast address, which all slave devices recognize.
3.3 Modbus functions
The Function Code field tells the addressed slave what function to perform. The following table lists those
functions supported by the Fieldbus board:
Code Name Meaning of Fieldbus board
01 READ COIL STATUS Read Control bits
02 READ INPUT STATUS Read Status bits
03 READ HOLDING REGISTER Read VACON parameter
04 READ INPUT REGISTER Read VACON variable
05 FORCE SINGLE COIL Write Control bits
06 PRESET SINGLE REGISTER Write VACON parameter
08 DIAGNOSTICS Test and checking of the communication system
3.4 Error detection
Communications errors usually consist of a changed bit or bits within a message. Communications errors
are detected by character framing, a parity check, and a redundancy check.
The MODBUS system provides several levels of error checking to assure the quality of the data
transmission. To detect multibit errors where the parity has not changed, the system uses redundancy
checks: Cyclical Redundancy Check, (CRC), for the RTU mode and Longitudinal Redundancy Check,
(LRC), for the ASCII mode. The Fieldbus board uses only RTU mode.