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CMS-660 SYSTEM
CIRCUIT MONITORING SYSTEM (CMS)
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Modbus
Error Codes
Modbus protocol defines a common way of error reporting. Every request (read or write) sent in unicast
mode is expected to return a value in packet of the same structure. In case of a message delivery error
(not a CRC problem but a message execution problem), the generated response contains a function
code with MSB (80h) set and a single byte representing the error code, called “exception code”.
The following default exception codes are available:
Data and Control Registers
A register is always a two-byte (16-bit) value, which can be interpreted as either signed or unsigned
values or which has a special format.
In case of data represented in more than one register the concatenated registers will contain
information with MSB in the lowest address and LSB in the highest address within concatenated
addresses.
Do not use registers other than those specified.
Note 1: Format of one-word register for current values
unsigned
= 16-bit unsigned integer notation, resolution 0.01 A
signed
= 16-bit signed integer notation, resolution 0.01 A
0000h…7FEFh = 0.00 … 327.51 A
8000h…FFFFh = -327.66 … -0.01 A
Code
Name
Description
01h
Illegal function
Function is not supported
02h
Illegal data address
Register address is out of control unit’s range, or trying to write into a
read only register
03h
Illegal data value
Value is out of range
04h
Slave device failure
Unrecoverable error occurred while the control unit was attempting to
perform the requested action, for example, time-out
06h
Slave device busy
Control unit is currently in User Interface Configuration Mode. Unable
to execute the requested action .
Special values
Meaning
7FF0h
Data pending, acquisition in progress
7FF1h … 7FFBh
Reserved
7FFCh
The sensor is known but not accessible at the moment
7FFDh
Data type TrueRMS / AC / DC / Temperature is disabled
7FFEh
Overload (beyond full range)
7FFFh
Forbidden (no sensor with ID xx)
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Values with special meanings
Note 2: Format of polarity register for DC currents
0000h = direct = DC current from bottom to top (connector) side is positive
1000h = reverse = DC current from top (connector) to bottom side is positive
