Mitsubishi MELSEC-A Series Reference Manual page 67

4. COMPOSITION OF A THREE-TIER SYSTEM
4.3 Data Link System when the Second Tier is MELSECNET/B and the Third Tier is MELSECNET
In the system that the second tier is a MELSECNET/B data link system and the third tier is
a MELSECNET data link system, the second tier is connected with shielded twisted pair
cables, and the third tier is connected with optical fiber cables or coaxial cables.
4.3.1 System configuration
Fig 4.4 shows the configuration of the three-tier system.
(a) Up to 31 local and remote I/O stations can be connected to a master station for
the second tier.
(b) For the third tier, up to 64 local and remote I/O stations can be connected to the
master station, which is the local station for the second tier.
Fig 4.4 Three-tier system when the second tier is MELSECNET/B data link system
POINT
(1) Remote I/O stations cannot be connected in MELSECNET
(2) MELSECNET/B data link system has no restriction on the order of stations
(including master stations).
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mode.