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02
–
Read Discrete Inputs (0x02)
22
–
Write Mask Register (0x16)
05
–
Write Single Coils (0x05)
23
–
Read/Write Multiple Registers (0x17)
07
–
Exception Status (0x07)
24
–
Read FIFO Queue (0x18)
12
–
Get Comm Event Log (0x0C)
43
–
Encapsulated Interface Transport (02B)
20
–
Read File Record (0x14)
Exception Responses
Illegal commands and register data addresses, etc. will result in an exception response. The re-
ply has the top bit of the Function Code set and contains a one-byte exception code. For exam-
ple, a read coil command (not supported on GPLP probes) to slave 0x01 will generate this re-
sponse:
01 81 01 CRC
L
CRC
H
which is interpreted as:
01 Slave Address
81 Function Code - Exception on command function 0x01
01 Exception Code (Illegal Function)
CRC
L
CRC
H
–
CRC bytes
Exception codes are:
01 Illegal Function
–
the command function is not supported.
02 Illegal Data Address
–
the register address does not exist.
03 Illegal Data Value
–
attempt to set an illegal data value, for example illegal Modbus
sensor address.
05 Acknowledged
–
command function received; sensor will now be busy processing the
command.
06 Slave Busy
–
response when sensor is busy, e.g. during measurement process.
Response Time-Out
Most command functions will respond immediately (after approximately 4-character times),
however the write register functions may take longer, approximately 100 ms. The Master must
wait for this time for the response and before sending new commands. See also the Measure-
ment Sequence section below for a description of the measurement processing time during soil
moisture and soil temperature measurements.
