Mitsubishi MELSEC-A Series Reference Manual page 302

10. TROUBLESHOOTING
(5)
(b) The loop status and loopback execution station are the same as the following
registers:
1) Loop status:......................... D9204
2) Loopback execution station:... D9205, D9206
Operation status of all slave stations (local and remote I/O stations)
The status of all of the slave stations in the system is displayed as follows:
"L" column: Describes status of present data link
"C": Communicating normally (D9224 to D9227)
"D":Disconnected due to communication stop (D9224 to D9227)
The possible causes to be disconnected are as follows:
1) The power supply to the disconnected station is OFF.
2) The disconnected station is reset.
3) An error that causes operation stop to the programmable
controller CPU.
4) A MELSECNET-compatible local or remote I/O station is
connected to a station number where MELSECNET -compatible
station (local station) is set with link parameters.
5) The station is disconnected due to loopback processing.
Check the loopback with the display of loop status of the data link
system described in (4).
6) Incorrect cable wiring (For the optical fiber cable, the IN/OUT
connection is incorrect.)
"P":Parameter communication with master station(D9228 to D9231)
Link parameter communication is executed only once at the time of
starting communications.
Possible causes to display "P" continuously are as follows:
1) A remote I/O station is connected to a station number set as the
MELSECNET mode local station with the link parameters.
2) A local station is connected to a station number set as a remote
I/O station.
3) A remote I/O station is connected to a station number set as a
MELSECNET-compatible local station in the MELSECNET
composite mode with link parameters.
"S" column: Displays present CPU operation status (D9212 to D9215)
"R": Run status
"S": Stop status
Only "R" is displayed for a remote I/O station.
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