9440300990 28-5
DECS-250 Modbus
®
Communication
values in the data block result in an error response message with an exception response code. The
exception response codes supported by the DECS-250 are provided in Table 28-3.
Table 28-3. Supported Exception Response Codes
01 Illegal Function The query Function/Subfunction Code is unsupported;
query read of more than 125 registers; query preset of
more than 100 registers.
02 Illegal Data
Address
A register referenced in the data block does not support
queried read/write; query preset of a subset of a numerical
register group.
03 Illegal Data Value A preset register data block contains an incorrect number
of bytes or one or more data values out of range.
DECS-250 Modbus
®
via Ethernet
Modbus can communicate through Ethernet if the IP address of the DECS-250 is configured as described
in the Communications chapter of this manual.
Detailed Message Query and Response for RTU Transmission Mode
A detailed description of DECS-250 supported message queries and responses is provided in the
following paragraphs.
Read Holding Registers
Query
This query message requests a register or block of registers to be read. The data block contains the
starting register address and the quantity of registers to be read. A register address of N will read holding
register N+1. If the query is a broadcast (device address = 0), no response message is returned.
Device Address
Function Code = 03 (hex)
Starting Address Hi
Starting Address Lo
No. of Registers Hi
No. of Registers Lo
CRC Hi error check
CRC Lo error check
The number of registers cannot exceed 125 without causing an error response with the exception code
for an illegal function.
Response
The response message contains the data queried. The data block contains the block length in bytes
followed by the data (one Data Hi byte and one Data Lo byte) for each requested register.
Reading an unassigned holding register returns a value of zero.
Device Address
Function Code = 03 (hex)
Byte Count
Data Hi (For each requested register, there is one Data Hi and one Data Lo.)
Data Lo
.
.
Data Hi
Data Lo
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