Technical Manual CODEL
32
Standard MODBUS Communication with CODEL MODBUS TT AQM
Summary
Using standard MODBUS protocol function 03 allows a host to obtain the contents of one or more
holding registers in the CODEL MODBUS TT 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 TT 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
Address of starting register
Number of consecutive
registers to be read
Slave Response (from CODEL MODBUS TT AQM)
Date from starting register
Standard baud rate - 4800
Bits per byte - 1 start bit
- 8 data bits (least significant sent first)
- 1 stop bit
- no parity
The MODBUS output from the TT AQM is at 4800 baud format: 1 start bit, 8 data bits, no parity, 1
stop bit.
The MODBUS output from the TT AQM will be connected via RS485.
The TT AQM comms is Standard Modbus protocol with RTU framing and only function code 03 is
supported.
The TT AQM will be a MODBUS slave device.
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