Page 276 - Electric Drives and Electromechanical Systems

Chapter 11 Cyber Physical systems and security 273

Within the industrial networking domain, the lack of standardisation is the major
problem in inter-computer communication. The large number of signal characteristics
and topologies is just part of the problem, as manufacturers of machine-tools or robotic
systems have developed their own instruction sets, memory access systems, and
communication protocols. A machine operating programme that works on one system
will not necessarily be useable on a system provided by a different manufacturer, even if
the languages are broadly similar. However, as will be discussed later in this section, sets
of standards for industrial systems have evolved, in part driven by manufacturers
themselves. Before discussing standards, an overview of the basic network concepts will
be undertaken.
11.1.1 Networking

The exchange of data over a network can be either asynchronous or synchronous,
depending on the protocol being used. In asynchronous communications, each data
word is sent as a separate message. Asynchronous communication is often adequate if
only two computers are connected. Online programming and monitoring are often
undertaken asynchronously with a single computer directly connected to a PLC, robot,
or other controller.
Synchronous communication data messages consist of a number of data words that
are preceded by a header containing information about the data ‘packet’ and followed by
a footer containing error-checking information. Synchronous communication is
appropriate where large amounts of data are to be transmitted quickly. In general, local
area networks (LANs) use synchronous data communications.
The RS-232 standard is perhaps the most commonly used serial communication
standard, the standard dating from the 1960s. The RS-232 standard dictates that a binary 1
should be sent as a negative 3e12 V, and a binary 0 as a positive 3e12 V. The standard also
specifies a 25-pin connector, even though only 3 pins are essential (ground, transmit, and
receive), Fig. 11.1A. However, RS-232 has several disadvantages, including its maximum
cable length for reliable communication is relatively short (typically 15 m, or when the
capacitance of the cable exceeds 2500 pF), has large voltage swing, uses large standard
connectors, no multipoint capability and limited multidrop capability. Due to these dis-
advantages it has largely disappeared from personal computing applications but is still
used in industrial systems due to its robustness.
The RS-485 standard can be used effectively over long distances up to 1,200 m and in
electrically noisy environments, as multiple receivers may be connected to the network in a
linear, multidrop bus configuration. In the RS-485 standard the line drivers use three-state
logic allowing individual transmitting nodes to be deactivated. This allows RS-485 to
implement linear bus topologies using only two wires, Fig. 11.1B. It is important to
incorporate a termination resistance, as the data lines float in such a way that electrical
noise sensitivity is greatest when all device stations are silent or unpowered. A significant
proportion of PLC local area networks currently use the RS 485 standard.