EXAMPLE 1 DATA REQUEST
The MASTER requests the Process Variable from SLAVE device 2. The process is
initiated by the MASTER sending the following message.
02<STX>?CH000<ETX><BCC>
In order to see what actual data is sent from the host, see the table below. Note that
the data is in hexadecimal.
All three units receive the message, although units 1 and 3 will disregard it as the 02
carried in the initial part of the message designates the message as being for device 2
only. Device 2, on receipt of the message recognises the 02 address as its own device
number and examines the main part of the message to see what action to take. The
main part of the message in this case is ?CH000. The question mark is the first
character and is used to denote that the message is a data request. CH is a mnemonic
representing CHannel value ( Process Variable ). A full list of mnemonics are
available later on in this Appendix. The remainder of the message is called the index,
and as there is only one Channel on this device it is superfluous and is set to zero.
Having recognised a valid message, the instrument first acknowledges back to the host
and then replies with the current Process Variable value. The format for the message
is shown below, notice there is no device number embedded in the message- there is
only one MASTER device.
APPENDIX E
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