Page 126 - Gas Detection Systems Training Manual

Implemented Functions and Registers Defined
The function code identifies the command being issued to the device. It is one byte in length and has a value of 1, 3, 5, 6, 15 or 16.
Table 6: Function Codes Supported By SEC 3120
For the current 16-bit version, registers are only mapped for function codes 3, 6, and 16. Otherwise an exception response will be generated.
In most cases, there is no response for a query that contains an invalid slave address, invalid CRC data or a non-supported broadcast mode, etc. In some cases, the unit might issue an Exception 2 as an error response if an invalid register address is requested for a valid function code. However, if invalid function code is issued, such as function 07 were transmitted, then an Exception 1 message would be generated to indicate that the function code is not supported.
Function Code
Description
1 Read Coil Status Registers (not supported)
3 Read Holding Registers
5 Force Single Coil Register (not supported)
6 Preset Single Holding Register
15 Force Multiple Coil Registers (not supported)
16 Preset Multiple Holding Registers
Query Coil Status Message
Field Name
Slave Address Function Code Address High Address Low Num Points High Num Points Low CRC Low Byte CRC High Byte
Example (Hex)
01 03 01 5B 00 03 75 E4
Response Exception 2
Field Name
Slave Address Function Code Exception Code CRC Low Byte CRC High Byte
Example (Hex)
01 83 02 C0 F1
SEC 3120 16-bit Modbus Manual Page 25 Of 46 Sensor Electronics Corporation For Version 1.2 and above December 2012 P/N 1580282, Rev. 1, 20121228