West Control Solutions Pro-EC44 - Supported Modbus Functions; Function Descriptions; Function 03; 04 - Read Holding;Input Registers; Function 06 - Write Single Register

Pro-EC44 2-Loop Graphical Profile Controller & Recorder
Pro-EC44 Product Manual - 59540-2 September 2014 Page 111
Supported Modbus Functions
The following Modbus function types are supported by this instrument:
Function
Code
Modbus Meaning Description
03 / 04 Read Holding/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 Write Single Register Writes two bytes to a specified word address.
08 Diagnostics Used for loopback test only.
16 (0x10 hex) Write Multiple Registers Writes up to 253 bytes of data to the specified address
range.
23 (0x17 hex) Read/Write Multiple Registers Reads and Writes 253 bytes of data to the specified
address ranges.
Function Descriptions
The following is interpreted from the Modbus protocol description obtainable from
www.modbus.org. Refer to that document if clarification is required. 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.
Function 03 / 04 - Read Holding/Input Registers
Reads current binary value of data at the specified word addresses.
QUERY: Function 03 / 04 - Read Holding/Input Registers
Func
Code
Address of
1
st
Word
Number of
Words
03/04 LO LO HI LO
RESPONSE: Function 03 / 04 - Read Holding/Input Registers
Func
Code
Byte
Count
1
st
Word etc Last Word
03/04
xx
HI LO → HI LO
Note: In the response the “Number of Bytes” indicates the number of data bytes
read from the instrument. E.g. if 5 words are read, the count will be 10 (0xA hex).
The maximum number of words that can be read is 64. If a parameter does not
exist at one of the addresses read, a value of 0000h is returned for that word.
Function 06 - Write Single Register
Writes two bytes to a specified word address.
QUERY: Function 06 - Write Single Register
Func
Code
Address of
Word
Value to write
06 HI LO HI LO
RESPONSE: Function 06 - Write Single Register
Func
Code
Address of
Word
Value Written
06 HI LO HI LO
Note: The Response normally returns the same data as the query.