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7 Using Modbus-RTU Slave Mode
7-12
CJ-series Serial Communications Units Operation Manual for NJ-series CPU Unit(W494)
z
Protocol Data Error
This bit will be set to TRUE if a checksum error is detected in the protocol data when the Controller
is reset. At the same time, the ERC indicator will flash. The checksum is checked for all serial com-
munications modes.
The operation of Modbus-RTU Slave communications is not affected by protocol data error.
z
Port Setting Status (System Settings and Hardware Settings)
The system settings for the following items will be stored: Serial Communications Mode and Communi-
cations Specifications, Hardware Settings for Port Types and Terminating Resistance ON/OFF, System
Settings Error, Port Operating/Stopped Status. The Port Operating Status will always be TRUE for Mod-
bus-RTU Slave Mode.
z
Communications Status
The flow control and buffer statuses are shown.
These statuses are not used in the Modbus-RTU Slave Mode.
These bits are cleared when the Controller is reset or when a port is restarted using the Port 1/Port
2 Port Settings Change Bits.
z
Communications Status
The status of the following transmission control signals is stored: DTR signal, DSR signal, CTS sig-
nal, and RTS signal (TRUE: High, FALSE: Low).
z
Transmission Error Status
The Transmission Error Flag (bit 15) will be changed to TRUE if any of the following flags is changed
to TRUE: CRC error (bit 07), Overrun Error (bit 04), Framing Error (bit 03), or Parity Error (bit 02).
z
Number of Normally Received Commands
The number of normal Modbus-RTU commands received from the communications port is stored.
z
Number of Normally Sent Responses
The number of normal Modbus-RTU responses sent to the communications port is stored.
z
Number of Overrun Errors, Framing Errors, and Parity Errors
The number of overrun errors, framing errors, and parity errors that occurred is stored.
z
Number of CRC Errors
The number of CRC errors that occurred is stored.
z
Number of Command Format Errors
The number of illegal function codes and illegal addresses in received Modbus-RTU commands is
stored.
Additional Information
The above counters (Number of Normally Received Commands, Number of Normally Sent
Responses, Number of Transmission Errors and Number of Command Format Errors) are
cleared to 0 when the Controller is reset. Counting will stop if they reach 65,535.