Page 277 - Electric Drives and Electromechanical Systems

274 Electric Drives and Electromechanical Systems

FIG. 11.1 Comparison between the RS-233 and RS-484 standards as applied to serial communication. (A) A RS-233
communication link, showing the basic interconnections; G is ground, Tx is data transmission, and Rx is data
receive with reference to the host computer. (B) A two-node interconnection using the RS-484 standard, when
one node is transmitting the other transmit nodes are deactivated.

For long-distance communication (over 1.5 km), the RS485 standards does not
provide the required capability. One commonly-used solution is to use fibre optic
networking instead of conventional copper. The Fibre Digital Data Integration (FDDI)
standard offers potential data transmission rates well into the hundreds of Mbps. One of
the significant advantages of a fibre optic link is its resilience to EMI, which is a common
cause of many problems within an industrial network.
The local area network (LAN) technology found in most industrial organisations is
based on the Ethernet protocol, which is formally specified in the IEEE 802.3 standard,
using coaxial cable or twisted pairs. A feature of the basic Ethernet is that the standard
makes no provision for power being supplied via the LAN, hence all hubs and peripherals
need to be independently powered; while this is not a problem with a computer, it could
be for industrial sensors. This matter was resolved by an addition of the Power-
over-Ethernet addendum as part of the IEEE Ethernet Standard. This provision
provides power up to 70 W to be used by the Ethernet sensors or similar devices. One
point that should be noted in all industrial networking is the careful selection of
connectors, the conventional RJ45 connector provides little of no mechanical or
environmental protection, hence specialist connectors need to be used to give the
required long term reliability.

11.1.2 Network topology
There are several architecture options for interconnection of cell or robot controllers,
together with central computers within the industrial environment, as shown in Fig. 11.2.