Page 699 - Mechanical Engineers' Handbook (Volume 2)

690 General-Purpose Control Devices

Table 6 Rated Values for Analog Inputs of Programmable
Controllers
Signal Range Input Impedance
10 to 10 V 10 k
0to 10 V 10 k
1to 5V 5k
4–20m A 300
Note: See the IEC Programmable Controller standard (Ref. 8) for ad-
ditional notes and rating values.

Table 7 Rated Values for Analog Outputs of Programmable
Controllers
Signal Range Load Impedance
10 to 10 V 1k
0to 10 V 1k
1to 5V 500
4–20 mA 600
Note: See the IEC Programmable Controller standard (Ref. 8) for ad-
ditional notes and rating values.

Communication interfaces for programmable controllers provide many different com-
binations of connectors, signal levels, signaling rates, and communication services. The man-
ufacturer’s specifications of these characteristics should be checked against applicable
standards to assure the achievement of the required levels of system performance and com-
patibility of all GPCDs in the system.

2.3 Programming
Programmable controllers have historically been used as programmable replacements for
relay and solid-state logic control systems. As a result, their programming languages have
been oriented around the conventions used to describe the control systems they have replaced,
that is, relay ladder logic and function block diagrams. Since these representations are fun-
damentally graphic in nature, programmable controllers provide one of the first examples of
the practical application of graphic programming languages.
Much of the programming of programmable controllers is done in the factory environ-
ment while the controlled system is being installed or maintained. Hence, programming was
traditionally supported by special-purpose portable programming terminals. In recent years,
most of the support for PLC programming has migrated to software packages for personal
computers. However, some need still exists for specialized terminals for programming and
debugging of programmable controllers in the industrial environment, although most of this
functionality can also be supplied by ruggedized, portable personal computers. The selection
of support environments is thus an important consideration in the selection and implemen-
tation of programmable controller systems.
3
1
The standard for PLC programming languages is published by the IEC. This standard
specifies a set of mutually compatible programming languages, taking into account the dif-