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15.5 Supported Modbus Functions
The following Modbus function types are supported by this instrument:
Function Code
decimal
(hexadecimal) Modbus Meaning Description
03 (0x03)
04 (0x04)
Read Holding Registers
Read Input Registers
Read current binary value of specified number of parameters at
given address.
Up to 64 parameters can be accessed with one query.
06 (0x06)
Write Single Holding
Register
Writes 2 bytes to a specified word address.
08 (0x08) Diagnostics Used for loopback test only to check the communications work.
16 (0x10)
Write Multiple Holding
Registers
Writes up to 253 bytes of data to the specified address range.
15.6 Function Descriptions
The following is interpreted from the Modbus Protocol Description obtainable from http://www.modbus.org/.
In the function descriptions below, the preceding device address value is assumed, as is the correctly formed two-
byte CRC value at the end of the QUERY and RESPONSE frames.
15.7 Function 03 / 04 - Read Holding/Input Registers
Reads the current binary value of data at the specified word addresses.
Query
Function Address of 1st Word Number of Words
03/04 HI LO HI LO
Response
Function Number of Bytes First Word Last Word
03/04 n HI LO HI LO
In the response, the “Number of Bytes” ‘n’, indicates the number of data bytes read from the instrument. E.g. if 5
words are read, the count will be 10 (A hex). The maximum number of words that can be read is 64. If a parameter
does not exist at one of the addresses read, then a value of 0000h is returned for that word.
Function 06 – Write Single Register
Query
Function Diagnostic Code Value
06 HI = 00 LO = 00 HI LO
Response
Function Sub-Function Value
06 HI = 00 LO = 00 HI LO
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