Mitsubishi MELSEC-A Series Reference Manual page 32

2. TWO-TIER SYSTEM IN THE MELSECNET DATA LINK SYSTEM
(3) Improved RAS (Reliability, Availability, Serviceability) functions
(a) Loopback function
If a cable breaks or the power supply to a slave station is turned off, the affected
slave station is disconnected from the data link so that normal link operations are
able to continue for other stations.
In the MELSECNET data link system, the optical fiber cables or coaxial cables
are doubled to make the loopback function possible. This double configuration
allows data link operations to continue by switching the loop from forward to
reverse if a cable breaks or the power to a slave station is turned off. (Refer to
Section 5.3.4.)
(b) Automatic return function
If an error occurs in a slave station, the corresponding station is disconnected from
the data link system.
When a slave station (local station, remote I/O station) is disconnected from the
link due to an occurrence of a problem, it is automatically connected into the link
when the station recovers the normal operating conditions. (Refer to Section
5.3.3.)
(c) Error detection
1) The data link operation status is stored in special relays (M) and special
registers (D) in a programmable controller CPU.
Read these special relays (M) and special registers (D) to check the data link
operation status.
2) Use the link monitor function of a peripheral device (A6GPP, A6PHP, A6HGP)
to check the data link operation status.
(d) Self-diagnostics function
The self-diagnostics function checks the link module hardware, the optical fiber
cable or coaxial cable connection status, etc.
REMARK
The errors that make the RAS functions valid are only cable breakage, slave station
power-off, data link setting error, and the errors that can be detected by the self-
diagnostics of the CPU module.
The RAS functions may not work depending on the fault of the data link module.
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