Siemens SITRANS F Modbus RTU RS 485 - Page 25

The data section of the response message contains a number of bits representing the status of
the coils(s) that has been read from the device. The LSB of the coil status byte indicates the status
of the coil.
Table 6-3 Read coil normal response messages
Message byte Example
Slave address xxhex
Function code 01hex
Bit count xxhex
Coil status byte(s) xxhex
xxhex
Next coil status byte(s) xxhex
xxhex
Last coil status byte(s) xxhex
xxhex
CRC-16 Lower xxhex
Upper xxhex
Exception response
The exception response message contains the same slave address as the command message.
The function code of the exception message is actually a value of 80 hex plus the original
function code of 01 hex .
The exception code indicates where the error occurred in the command message. A complete
listing of exception codes is shown in a later chapter.
Table 6-4 Read coil exception response messages
Message byte Example
Slave address xxhex
Function code 81hex
Exception code 02hex
CRC-16 Lower xxhex
Upper xxhex
Example
Read Coil Totalizer 1 (0:00011):
• Query: 1,1,0,10,0,1,221,200 (Hex 01,01,00,10,00,01,DD,C8)
• Response: 1,1,1,0,81,136 (Hex 01,01,01,00,51,88)
Functions codes
6.2 Read coils
Modbus RTU RS 485
Operating Instructions, 03/2021, A5E00753974-AB 25