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102
© Servomex Group Limited. 2020 01910001B_8
A.5 Exception codes
If an error should occur while processing a message one of the following excep�on codes will be
returned by the instrument.
Code Condition Meaning
01
Illegal func�on
Requested func�on co
de is not
supported.
02
Illegal data address
The combina�on of data
address and transfer length is
invalid for this func�on.
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 par�cular register.
04 Slave device failure
An
unrecoverable error
occurred while the unit was
atemp�ng to perform the
requested ac�on.
A.6 Addressing
Addresses
in Modbus ADU (applica�on 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 par�cular 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.
A.7 Floating point numbers
Floa�ng point numbers (e.g. 12.34,
-
1012.32, etc.), are digitally represented using the IEEE
-
754 format.
Single precision floa�ng point numbers are used throughout and they require 32 bits of data. Since a
Modbus register holds 16 bits it takes 2 registers to rep
resent a
floa�ng
point
number. We default to
having the most significant word of the float, bits 16
-
31, in the first register, and the least significant
word, bits 0
-
15, in the next register. This order can be reversed by se�ng a coil.
A.8 System data mapping
Read-
only access to system data will be provided in a block of 100 registers. These can be accessed as
input registers using func�on code 04.