MultiExact Gas Analyser
Appendix
A3.4 Supported function codes
For simplicity, only the following function codes will be supported.
Function
Description
Usage
01
Read coils
Read calibration status, pump state, etc.
02
Read discrete inputs
Read faults and alarm states.
03
Read holding registers
Read settings.
04
Read input registers
Read measurements, units, etc.
05
Write single coil
Change modes, perform calibration etc.
06
Write single register
Change single setting.
08
Sub Function
00 = Return
query data
Diagnostic to test communications.
16
Write multiple registers
Change multiple settings.
A3.5 Exception codes
If an error should occur while processing a message one of the following exception
codes will be returned by the instrument.
Code
Condition
Meaning
01
Illegal function
Requested function code is not supported.
02
Illegal data address
The combination of data address and transfer
length is invalid for this function.
03
Illegal data value
A value contained in the query data field is not an
allowable value. This indicates a fault in the
structure of the remainder of a complex request.
This does NOT mean that a value to be stored in a
register is incorrect as Modbus has no means of
determining what is legal for any particular register.
04
Slave device failure
An unrecoverable error occurred while the unit was
attempting to perform the requested action.
A3.6 Addressing
Addresses in Modbus ADU (application data unit), run from 1 – N, whereas addresses
in the Modbus PDU (protocol data unit) run from 0 – N. This appendix gives addresses
in the ADU model. Depending on the particular Modbus master, addresses may have
to be entered as they are given or have 1 subtracted from them. For example to read
register 101 an address of 100 may be needed.
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