Despatch Protocol 3 Modbus Communications - Manual (Page 9)

Modbus Programming Manual A BOUT T HIS M ANUAL
Version 2 9
Copyright © 2012 by Despatch Industries.
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Inter-message
gap Address
1 character Function
1 character Data
n characters CRC Check
2 characters
1.4.6. Supported Modbus Functions
Modbus defines several function types. The following types are supported by this instrument:
Function Code
(decimal) 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.
1.4.7. Function Descriptions
The following is interpreted from the Modbus Protocol Description obtainable from
http://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 Address of 1 st Word Number of Words
03 / 04 HI LO HI LO
RESPONSE
Function Number of Bytes First Word Last Word
03 / 04 HI LO HI LO
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 (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.