BECKHOFF BC7300 - Manual (Page 28)

MODBUS
28 BC7300
Byte Name Example RTU ASCII
Start frame „:“ 0x3A
Slave address 11 0x0B „0B“ 0x30, 0x42
Function code 1 0x01 „01“ 0x30, 0x31
Byte Count 2 0x02 „02“ 0x30, 0x32
Data bits 0..7 4 0x04 „04“ 0x30, 0x34
Data bits 8..15 0 0x00 „00“ 0x30, 0x30
Error Check
LRC / CRC
0x23
0x3D „EE“ 0x45, 0x45
End of frame t1-t2-t3 CRLF 0xD, 0xA
Read Digital Inputs (Function 2)
READ INPUT STATUS
Function 2 can be used to read the digital input data.
In this example the first 10 digital inputs of slave number 11 are read. The
start address is zero. If an offset is to be entered, this is done in the "Start
address" field.
Query
Byte Name Example RTU ASCII
Start frame „:“ 0x3A
Slave address 11 0x0B „0B“ 0x30, 0x42
Function code 2 0x02 „02“ 0x30, 0x32
Start address high 0 0x00 „00“ 0x30, 0x30
Start address low 0 0x00 „00“ 0x30, 0x30
Count high 0 0x00 „00“ 0x30, 0x30
Count low 10 0x0A „10“ 0x31, 0x30
Error Check
LRC / CRC
0xF8
0xA7 „E3“ 0x45, 0x33
End of frame t1-t2-t3 CRLF 0xD, 0xA
Response
The Bus Terminal Controller answers with byte count 2, i.e. 2 bytes of data
are returned. The query was for 10 bits, and these are now distributed over
2 bytes. The first bit in the input process image of the BC7300 is set, and
the Bus Coupler returns a "1" in the first data byte.
Byte Name Example RTU ASCII
Start frame „:“ 0x3A
Slave address 11 0x0B „0B“ 0x30, 0x42
Function code 2 0x02 „02“ 0x30, 0x32
Byte Count 2 0x02 „02“ 0x30, 0x32
Data 0..7 1 0x01 „01“ 0x30, 0x31
Data 8..15 0 0x00 „00“ 0x30, 0x30
Error Check
LRC / CRC
0x20
0x29 „F0“ 0x46, 0x30
End of frame t1-t2-t3 CRLF 0xD, 0xA
Read Analog In/Outputs (Function 3)
1
READ HOLDING
REGISTERS
Function 3 can be used to read the input and output words and the
registers.
In this example the first two analog outputs of slave number 11 are read.
The analog outputs begin at offset 0x800 (hex). The length indicates the
number of channels to be read.
1
From Firmware B4 reading with function 3 is allowed.