C1300
Advanced Circular Chart Recorder
5 Modbus Functions
IM/C1300–MOD
Rev. C
11
5.4 Preset Single Register – Function Code 06
5.4.1 Preset Single Register Query
The preset single register allows the user to modify the contents of a holding register.
Example
– a preset single register request to write the value 500 to holding register address 121 (alarm trip A1) in slave 01 is shown
below.
5.4.2 Preset Single Register Response
The normal response to a preset single register request is to retransmit the query message after the register has been altered.
Example:
5.5 Loopback Test – Function Code 08
5.5.1 Loopback Test Query
The purpose of the loopback test is to test the Modbus system, it does not affect the content of the controller. Variations in the
response may indicate faults in the Modbus system. The information field contains 2 bytes for the designation of the diagnostic code
followed by 2 bytes to designate the action to be taken.
Example:
*These are considered to be the information fields for diagnostic mode.
5.5.2 Loopback Test Response
The response always echoes the query, only diagnostic code 0 (bytes 3 and 4) can be used.
Example:
*These are considered to be the information fields for diagnostic mode.
Note.
Function codes 5, 6 and 16 are the only messages that are recognized as valid for broadcast.
Note.
To write to a register, the register's offset address must be used, e.g. to write to register 121, the offset address 120(78H)
is transmitted.
Address
Function
Register Offset
Data Value
Error Check Field
(CRC-16)
High
Low
High
Low
01
06
00
78
01
F4
09
C4
Address
Function
Register Offset
Data Value
Error Check Field
(CRC-16)
High
Low
High
Low
01
06
00
78
01
F4
09
C4
Address
Function
Data Diagnostic Code
Data*
Data*
Error Check Field
(CRC-16)
High
Low
01
08
00
00
A5
37
DA
8D
Address
Function
Data Diagnostic Code
Data*
Data*
Error Check Field
(CRC-16)
High
Low
01
08
00
00
A5
37
DA
8D
