Page 697 - Mechanical Engineers' Handbook (Volume 2)

688 General-Purpose Control Devices

and from top to bottom. Programming elements are sometimes provided to enable
skipping the evaluation of groups of program elements or to force the outputs of
a group of elements to zero.
c. Evaluating transition conditions of the SFC (if any) at the end of the program
scan, in preparation for step 2a in the next program scan.
3. The data from the updated output image are then transferred to the interfaces and
closed-loop controllers on the I/O bus and possibly to other GPCDs as well.
4. Finally, ‘‘housekeeping’’ tasks are performed on a time-available basis. These typi-
cally include communication with the operator, a supervisory controller, a program-
ming terminal, or other GPCDs.
After the performance of housekeeping tasks, the cyclic operation of the PLC begins again
with the input scan. This may follow immediately upon execution of the housekeeping tasks
or may be scheduled to repeat at a fixed execution interval.
Some programmable controller systems with separate I/O and/or communications proc-
essors provide for overlapping the scanning of the user program with the scanning of the
inputs (step 1) and outputs (step 3) and communication functions (step 4). In these cases,
special programming mechanisms may be needed to achieve concurrency and synchroniza-
tion between the program and I/O scans and between the program and communications
processing.
A further feature of some PLCs is the incorporation of a mutitasking operating system.
As in general computing, the PLC operating system serves to coordinate the multitude of
hardware and software resources and capabilities of the PLC. The incorporation of a mul-
titasking operating system serves to allow the PLC to essentially execute multiple instances
of the basic operation cycle shown in Fig. 9, rather than a single instance, at the same time.
This affords much increased flexibility and capability over a single-tasking operating system
PLC.
In PLCs incorporating multitasking operating systems, mechanisms may be needed to
achieve appropriate levels of task coordination and synchronization between the multiple
instances of the basic operating cycle. As an example, one task’s results may affect another
task’s decisions, especially with regard to I/O.

2.2 Interfaces
The IEC has specified the standard voltage ratings shown in Table 2 for power supplies,
8
digital inputs, and digital outputs of programmable controllers. The IEC standard also de-
fines additional parameters for digital inputs and outputs, shown in Tables 3–5; the param-
eters specified by the manufacturer should be checked against those defined in the IEC
standard in order to assure the suitability of a particular input or output module for its
intended use in the control system.
The IEC-specified signal ranges for analog inputs and outputs for programmable con-
trollers are shown in Tables 6 and 7, respectively. The IEC standard lists a number of
characteristics whose values are to be provided by the manufacturer, such as input impedance,
maximum input error, and conversion time and method, and which must be checked against
the requirements of the particular control application.
In addition to simple digital and analog inputs and outputs, closed-loop controllers which
can reside on the I/O bus of the programmable controller system may be provided, as
illustrated for GPCDs in Fig. 2. In this case, the programming languages for the program-
mable controller typically provide language elements, in addition to those described in Sec-
tion 2.3, to support the configuration and supervisory control of these ‘‘slave’’ closed-loop
controllers.