Read Coils - Siemens SITRANS F Operating Instructions Manual

Functions codes

6.2 Read coils

6.2 Read coils
Command messages
The read coils functions (01hex x) allow the master to request information from the slave.
The command message of a coil read is structured as shown below. Each row in the table
compares to a byte in the message – top byte (Slave address) is transmitted first.
The initial slave address is 1. Address 0 (broadcast) and other values between 1 and 255
can be used.
The function code of this message is 01 hex (read coil) .
The starting coil is the first binary data to be read.
The quantity indicates how many consecutive bits are to be read. The quantity may range
from 1 to 432 bits.
A CRC value is generated from a calculation using the values of the slave address, function
code, and data sections of the message. When the slave receives the command message it
calculates a CRC-16 value and compares it to the one in the CRC-16 field of the command
message. If these two CRC-16 values are the same the slave has received the proper
command message. If the two CRC-16 values are not the same the slave will not respond.
Table 6- 2
Message byte
Slave address
Function code
Starting coil
Quantity
CRC-16
Normal response
If the command message has a valid slave address, function code, starting coil and quantity
value the slave will respond with a normal response message. If the command message has
an invalid slave address, function code, starting coil and/or quantity the slave will respond
with an exception response message.
The normal response message contains the same slave address and function code as the
command message.
The Bit count is the number of status bits returned in the response message. The number
equals the quantity in the command message.
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.
24
Read coil command messages
Upper
Lower
Upper
Lower
Lower
Upper
Example
xxhex
01hex
00hex
00hex
00hex
xxhex
xxhex
xxhex
Operating Instructions, 06/2010, A5E00753974-04
Modbus RTU RS485