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After processing the received command, the instrument will send a response to the
PC, transmitting the requested data or only confirming the command execution if data
transfer has not been requested. The response structure is identical to that of the
message sent by the "Master" unit.
• Address: address of answering transmitter.
Length = 1 byte
• Function: type of operation performed by transmitter.
Length = 1 byte
• Data:
Data communicated by transmitter to PC.
Length = 0 to 252 byte
• CRC:
CRC Control code.
Length = 2 bytes
When value "0" is the address field, it means that the command has been sent to all
transmitters in the network. In this case, instruments execute the command but don't
send any response to PC, in order to prevent transmission conflicts between
instruments. A command that is simultaneously sent to all instruments will be
therefore a parameter setup command, but not a request of data.
In order to prevent transmission conflicts between instruments, the PC shall also have
to wait for response from the questioned instrument before sending a new command.
If the instrument doesn't receive a correct command (time intervals between
characters or control codes not respected), it will not reply to the PC. If the PC doesn't
receive any response within a specified time interval (time-out), it will consider the
command receipt by the receiver as not successful and will try again the transmission
or will generate an error signal.
In appendix A, the CRC control code computing method is explained along with the
use of the parity bit.
Functions that can be requested by the PC to the instrument, with their codes to be
inserted in the command Function field, are described in the following table:
Tab. 8.A: MODBUS functions
Function code
01h
03h
04h
05h
06h
07h
10h
2Bh
Relay state reading
Configuration parameters reading
Measurements reading
Permanent storage of changed parameters
Single configuration parameter setup
Error conditions reading
Consecutive configuration parameter setup
Instrument general information reading
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Function
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