User Manual

Commissioning
Part No.: 4417.762_Rev07 SmartRadar FlexLine
Service Manual 6 - 9
A typical message frame is shown below:
 Start: Synchronisation 3.5 character time elapsed.
 Address: The address field of a message frame contains eight bits.
The address must be within the range 1 to 247 (decimal). A master
addresses a slave by placing the slave address in the address field
of the message. When the slave sends its response, it places its own
address in this address field of the response to let the master know
which slave is responding.
 Function: The function code field of a message frame contains eight
bits. With the Modbus protocol only function code 03 (read
holding registers) and function code 05 (force single coil) are
supported. When a message is sent from a master to a slave the
function code field tells the slave what kind of action to perform. For
a normal response, the slave simply echoes the original function
code.
 Data: The data field is constructed using sets of two 8 bit bytes (16
bit registers). The data field of messages sent from a master to slave
devices contains additional information which the slave must use to
take the action defined by the function code. The data field of a
response from a slave to a master contains the requested data.
 CRC check: The CRC check field contains a 16-bit value imple-
mented as two eight-bit bytes. The error check value is the result of a
CRC (Cyclical Redundancy Check) calculation performed on the
message contents. The CRC field is appended to the message as
the last field in the message.
 End: Synchronization 3.5 character time elapsed.
6.3.3.2.1 Function Codes
 Function code 03: Read holding registers
Holding registers are located in the memory range (4)0000 ... (4)
FFFFH. Register values can range from 0000 to FFFFH. Depending
on the use, the registers contain a value or bit coded status in single
(16 bit) or double (32 bit) register signed (two=s complement) or not
signed.
• The query message specifies the starting register and quantity of
registers to be read:
Start Address Function Data
CRC
check
End
T1T2T3T4 8 bits 8 bits n * 8 bits 2 * 8 bits T1T2T3T4
Slave
address
Function 03
Start
address
No. of
registers
CRC check
8 bits 8 bits 16 bits 16 bits 16 bits