Technical data

Modbus Message Format Abbreviations (continued)Table D-1:
LF ASCII line feed character (ASCII Modbus only)
LRC 8-bit Longitudinal Redundancy Check (ASCII Modbus only)
LSB least-significant byte
MSB most-significant byte
REG_CNT register count
START_REG starting register number
TFRAME three and one-half character time delay (RTU Modbus only)
Modbus Register Data per Format TypeTable D-2:
Integer (1 Modbus register) MSB, LSB
(IEEE) Floating Point (2 Modbus registers) (one exponent byte, three mantissa
bytes)
EXP,MAN, First Register
MAN,MAN, Second Register
Long Integer (2 Modbus registers)
MSB, MIDH, First Register
MIDL, LSB, Second Register
D.1.1 ASCII Modbus Format
Framing in ASCII transmission mode is accomplished by the use of the colon (:) character
to indicate the beginning of a frame and the carriage return/linefeed pair (CR/LF) to
indicate the end of a frame. The communication parameters for ASCII Modbus are 7 data
bits, even parity, and one stop bit.
The ASCII Modbus 8-bit Longitudinal Redundancy Check (LRC) is produced by converting
the hex characters to binary, adding the binary characters without wrap-around carry, and
two's complementing the result. The colon, carriage return and line feed characters are
not included in the LRC calculation.
The ASCII Modbus formats for the supported Modbus functions are described below.
Function Code 3 – Read Multiple Registers
Send Format:
• :, ADDR, FC, START_REG MSB, START_REG LSB, REG_CNT MSB,
• REG_CNT LSB, LRC, CR, LF
Response Format:
• :, ADDR, FC, BYTE_CNT, ...DATA... , LRC, CR, LF
Modbus communications
122 Daniel 3810 Series Operations Manual