Beijer Electronics NA-9379 - Manual (Page 62)

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MODBUS Programmable I/O NA-9379 FnIO S-Series
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A.3.3. 3 (0x03) Read Holding Registers
This function code is used to read the contents of a contiguous block of holding registers in a remote device. The
Request PDU specifies the starting register address and the number of registers.
The register data in the response message are packed as two bytes per register, with the binary contents right
justified within each byte. For each register, the first byte contains the high order bits and the second contains the
low order bits.
• Request
Field name Example RTU ASCII ASCII (bus line)
Start of Frame - t1-t2-t3 “,” 0x3A
Slave Address 0x07 0x07
“07”
0x30, 0x37
Function Code 0x03 0x03 “03” 0x30, 0x33
Starting Address Hi 0x08 0x08 “08” 0x30, 0x38
Starting Address Lo 0x00 0x00
“00”
0x30, 0x30
Quantity of Register Hi 0x00 0x00 “00” 0x30, 0x30
Quantity of Register Lo 0x02 0x02 “02” 0x30, 0x32
Error Check (CRC/LRC) - 0xC6, 0x0D
“EC”
0x45, 0x43
End of Frame - t1-t2-t3 CR, LF 0x0D, 0xA
• Response
Field name Example RTU ASCII ASCII (bus line)
Start of Frame - t1-t2-t3 “,” 0x3A
Slave Address 0x07 0x07
“07”
0x30, 0x37
Function Code 0x03 0x03 “03” 0x30, 0x33
Byte Count 0x04 0x04 “04” 0x30, 0x34
Output Register #0 Hi 0x11 0x11
“11”
0x31, 0x31
Output Register #0 Lo 0x22 0x22 “22” 0x32, 0x32
Output Register #1 Hi 0x33 0x33 “33” 0x33, 0x33
Output Register #1 Lo 0x44 0x44
“44”
0x34, 0x34
Error Check (CRC/LRC) - 0x2D, 0xC6 “38” 0x33, 0x38
End of Frame - t1-t2-t3 CR, LF 0x0D, 0xA
- In case of address 0x0800, 0x0801 output register value: 0x1122, 0x3344.