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Communications Timing Section 9-4
Error Format
9-4 Communications Timing
This section describes the timing of sending commands and controlling com-
munications in the Modbus-RTU Slave mode.
9-4-1 Simultaneous Commands and Communications Ports
The CS/CJ-series CPU Units provide eight logical ports that can be used
when executing SEND(090), RECV(098), and CMND(490). Therefore, up to
eight commands can be executed for one physical port during any one cycle.
The maximum number of messages, however, that can be sent or received in
one CPU Bus Unit service time is two messages from the CPU Unit to the
Board or Unit and two messages from the Board or Unit to the CPU Unit.
1514131211109876543210
D1000
3 A C 5
D1001
9 7 1 3
Length Data
Function Code 1 byte Function code + 80 hex
Exception Code 1 byte Error code
Error
code
Name Description
01 Illegal function • There is an error in the specified function code.
• A function code not supported by the Serial Com-
munications Board/Unit was specified.
02 Illegal data address • There is an error in the specified starting address.
• The combination of the specified starting address
and data length (quantity of registers/coils) exceeds
the valid range.
• The specified data length (quantity of registers/
coils) exceeds the valid range.
• The specified data length (quantity of registers/
coils) differs from the actual data length.
• Less than 4 bytes of data was received.
03 Illegal data value • There is an error in the specified data. (The data for
the Write Single Coil Command is not 0000 hex or
FF00 hex.)
04 Slave device failure • Reading or writing could not be performed because
an error occurred in the CPU Unit.
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