Read Input Status (Function 2)
The Read Input Status function can be used to read the digital input data. The first 10 digital inputs are read in this example. The start address is zero. An offset can be entered in the Start address field
Query
|
Byte Name |
Modbus RTU |
Modbus ASCII | ||
|---|---|---|---|---|
|
Start - Frame |
- |
- |
":" |
0x3A |
|
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 - Frame |
|
t1-t2-t3 |
CRLF |
0x0D, 0x0A |
The fieldbus coupler answers with Byte count 2, i.e. two bytes of data are returned. The query was for 10 bits, and these are now distributed over 2 bytes. The third bit in the output process image of the Coupler is set, and the fieldbus coupler returns the value 4 in the first data byte.
Response
|
Byte Name |
Modbus RTU |
Modbus ASCII | ||
|---|---|---|---|---|
|
Start - Frame |
- |
- |
":" |
0x3A |
|
Address |
11 |
0x0B |
"0B" |
0x30, 0x42 |
|
Function-Code |
2 |
0x02 |
"02" |
0x32, 0x30 |
|
Byte Count |
2 |
0x02 |
"02" |
0x32, 0x30 |
|
Data Bits 0..7 |
1 |
0x01 |
"01" |
0x31, 0x30 |
|
Data Bits 8..18 |
0 |
0x00 |
"00" |
0x30 , 0x30 |
|
Error Check, LRC/CRC |
|
0x20, 0x29 |
"F0" |
0x46, 0x30 |
|
End -Frame |
|
t1-t2-t3 |
CRLF |
0x0D, 0x0A |