The applications for industrial process-control systems are diverse,
ranging from simple traffic control to complex electrical power
grids, from environmental control systems to oil-refinery process
control. The intelligence of these automated systems lies in their
measurement and control units. The two most common computerbased
systems to control machines and processes, dealing with the
various analog and digital inputs and outputs, are programmable
logic controllers1 (PLCs) and distributed control systems2 (DCS’s).
These systems comprise power supplies, central processor units
(CPUs), and a variety of analog-input, analog-output, digitalinput,
and digital-output modules.
The standard communications protocols have existed for many
years; the ranges of analog variables are dominated by 4 mA
to 20 mA, 0 V to 5 V, 0 V to 10 V, ±5 V, and ±10 V. There
has been much discussion about wireless solutions for nextgeneration
systems, but designers still claim that 4 mA to 20 mA
communications and control loops will continue to be used for
many years. The criteria for the next generation of these systems
will include higher performance, smaller size, better system
diagnostics, higher levels of protection, and lower cost—all factors
that will help manufacturers differentiate their equipment from
that of their competitors.