Read or Write Registers (Function 23)
A number of analog outputs can be written and a number of analog inputs read with one telegram using the Read / Write Registers function. In this example the first two analog output words are written, and the first two analog inputs are read. The analog outputs start at offset 0x0800, while the inputs start at offset 0x0000. Here the offset always describes a word. Offset 0x0003 writes to the fourth word in the output process image. The length indicates the number of words, and the Byte count is formed from the combination of all the bytes that are to be written. Example: 4 words – correspond to a byte count of 8.
The data bytes contain the values for the analog outputs. In this example, two words are to be written. The first word is to receive the value 0x3FFF, and the second word is to receive the value 0x7FFF.
Query
|
Byte Name |
Modbus RTU |
Modbus ASCII | ||
|---|---|---|---|---|
|
Start - Frame |
- |
- |
":" |
0x3A |
|
Address |
11 |
0x0B |
"0B" |
0x30, 0x42 |
|
Function-Code |
23 |
0x17 |
"17" |
0x31, 0x17 |
|
Read Start-Address high |
0 |
0x00 |
"00" |
0x30, 0x30 |
|
Read Start-Address low |
0 |
0x00 |
"00" |
0x30, 0x30 |
|
Read Länge high |
0 |
0x00 |
"00" |
0x30, 0x30 |
|
Read Länge low |
2 |
0x02 |
"02" |
0x30, 0x32 |
|
Write Start-Address high |
8 |
0x08 |
"08" |
0x30, 0x38 |
|
Write Start-Address low |
0 |
0x00 |
"00" |
0x30, 0x30 |
|
Write Länge high |
0 |
0x00 |
"00" |
0x30, 0x30 |
|
v Länge low |
2 |
0x02 |
"02" |
0x30, 0x32 |
|
Byte Count |
4 |
0x04 |
"04" |
0x30, 0x34 |
|
Data 1 high |
63 |
0x3F |
"3F" |
0x33, 0x46 |
|
Data 1 low |
255 |
0xFF |
"FF" |
0x46, 0x46 |
|
Data 2 high |
127 |
0x7F |
"7F" |
0x37, 0x46 |
|
Data 2 low |
255 |
0xFF |
"FF" |
0x46, 0x46 |
|
Error Check, LRC/CRC |
|
0x76, 0xD3 |
"12" |
0x31, 0x32 |
|
End - Frame |
|
t1-t2-t3 |
CRLF |
0x0D, 0x0A |
Response
The coupler replies with the start address and the length of the bytes to be transferred in Byte count. The data information follows. In this example the first word contains 0x0038 while the second word contains 0x3F0B.
|
Byte Name |
Modbus RTU |
Modbus ASCII | ||
|---|---|---|---|---|
|
Start - Frame |
- |
- |
":" |
0x3A |
|
Address |
11 |
0x0B |
"0B" |
0x30, 0x42 |
|
Function-Code |
23 |
0x17 |
"17" |
0x31, 0x17 |
|
Byte Count |
4 |
0x04 |
"04" |
0x30, 0x34 |
|
Data 1 high |
0 |
0x00 |
"00" |
0x30, 0x30 |
|
Data 1 low |
56 |
0x38 |
"38" |
0x33, 0x38 |
|
Data 2 high |
63 |
0x3F |
"3F" |
0x33, 0x46 |
|
Data 2 low |
11 |
0x0B |
"0B" |
0x30, 0x42 |
|
Error Check, LRC/CRC |
|
0xF8, 0xA7 |
"E3" |
0x45, 0x33 |
|
End - Frame |
|
t1-t2-t3 |
CRLF |
0x0D, 0x0A |