Read Coil Status (Function 1)
The Read Coil Status function can be used to read the digital outputs that have been set.
The first 10 digital outputs 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 |
1 |
0x01 |
"01" |
0x30, 0x31 |
|
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 |
0x10 |
"10" |
0x31, 0x30 |
|
Error Check, LRC/CRC |
|
0xBC, 0xA7 |
"E4" |
0x45, 0x34 |
|
End - Frame |
|
t1-t2-t3 |
CRLF |
0x0D, 0x0A |
The fieldbus coupler 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 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 |
1 |
0x01 |
"01" |
0x30, 0x31 |
|
Byte Count |
2 |
0x02 |
"02" |
0x30, 0x32 |
|
Data Bit 0...7 |
4 |
0x04 |
"04" |
0x30, 0x34 |
|
Data Bit 8...18 |
0 |
0x00 |
"00" |
0x30, 0x30 |
|
Error Check, LRC/CRC |
|
0x23, 0x3D |
"EE" |
0x45, 0x45 |
|
End -Frame |
|
t1-t2-t3 |
CRLF |
0x0D, 0x0A |