CODEL Operation and Maintenance Manual
Issue : A Rev. : 3 Date : 13/11/2018 Ref. : 100351
P a g e | 34
Standard MODBUS Communication with CODEL MODBUS
Summary
Using standard MODBUS protocol function 03 allows a host to obtain the contents of one or more holding
registers in the CODEL MODBUS Air Quality Monitor (AQM). The request frame from the host (typically a DCS
or SCADA) defines the relative address of the first holding register followed by the total number of consecutive
registers to be read.
The response frame from the CODEL AQM lists the contents of the requested registers, returning 2 bytes per
register with the most significant byte first. A maximum of 125 registers can be accessed per request. The
formats of the request and response frames are detailed below, where ‘X’ and ‘n’ are hexadecimal variables.
An example of a MODBUS register map is shown below.
Host Request Frame
01 Address
03 Function code
XX
XX
Address of starting register
00
XX
Number of consecutive registers
to be read
XX
XX
CRC
Slave Response (from CODEL MODBUS AQM)
01 Address
03 Function code
XX Byte count
XX
XX
…to… …to…
Date from starting register
XX
XX
Date from nth register
XX
XX
CRC
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