Specifications

63
Schneider Electric
The Modbus Protocol (continued)
Definition: A bit is a basic unit of information that can only equal 1 or 0. Bits are the
"language" used by computers. They correspond to the following levels of electricity
in computers:
b "bit 0" = 0 Volt
b "bit 1" = 5 Volts
Important:
When bits are transmitted through a serial port, they correspond to the following
levels of electricity:
b bit 0 = +3V to +15V
b bit 1 = -3 to -15V
Definition: A byte is a group of eight (8) binary elements. A byte is thus eight (8)
bits that represent a binary "word".
Note: MSB = Most Significant Bit and LSB = Least Significant Bit.
Read n bits: function 1 or 2
b Query.
b Response.
Byte in detail:
Note: Unused bits in the byte are reset to zero.
Example:
Read bits 204 to 211 of slave no. 1.
b Query.
b In hexadecimal, the number of bits to read from 204 to 211 are as follows:
v 0204, 0205, 0206, 0207, 0208, 0209, 020A, 020B, 020C, 020D, 020E, 020F, 0210,
0211,
v a total of 14 values, or 0E in hexadecimal.
b Response.
b Function 1: read N output or internal bits.
b Function 2: read N input bits.
b The number of bits to read should be y
2000.
E52360E52361E52355E52362E52363
Slave no. 1 or 2
Address of the
1st bit to read:
Number of bits
to read:
1
≤
≤ n
≤
≤ 2000
CRC 16
MSB* LSB* LSB* MSB
*
1 byte 2 bytes 2 bytes 2 bytes
Slave no.
1 or 2
Number of
bytes read
1 byte
1 byte 1 byte n bytes 2 bytes
First byte read
CRC 16
Last byte
read
LSB* MSB
*
First bit
transmitted
Last bit
transmitted
01 01 02 04 010E
CRC 16
20B 204 211 20C
01 01 02
10101001
CRC 16
00101110
Guide to Integration
Modbus / Jbus