Page 219 - Programmable Logic Controllers, Fifth Edition - Mobile version

L1 Inputs Ladder logic program Output
Stop Start M L2
OL
Stop
M M

Start Starter
auxiliary Main power
contact contact
Starter
auxiliary
contact

Auxiliary
contact

Figure 9-24 Motor starter programmed using the starter auxiliary seal-in contact.
Source: Image Courtesy of Rockwell Automation, Inc.

Another safety consideration concerns the wiring of 9.8 Selectable Timed Interrupt
stop buttons. A stop button is generally considered a
safety function as well as an operating function. As such, The selectable timed interrupt (STI) instruction is used
all stop buttons should be wired using a normally closed to interrupt the scan of the main program file automati-
contact programmed to examine for an on condition cally, on a time basis, to scan a specified subroutine file.
(Figure 9-25). Using a normally open contact programmed For Allen-Bradley SLC 500 controllers, the time base at
to examine for an off condition will produce the same which the program file is executed and the program file
logic but is not considered to be as safe. Assume that the assigned as the selectable timed interrupt file are deter-
latter configuration is used. If, by some chain of events, mined by the values stored in words S:30 and S:31 of the
the circuit between the button and the input point were to status section of the data files. The value in S:30 stores
be broken, the stop button could be depressed forever, but the time base, which may be from 1 through 32,767, at
the PLC logic could never react to the stop command be- 10 millisecond increments. Word S:31 stores the pro-
cause the input would never be true. The same holds true gram file assigned as the selectable interrupt file, which
if power were lost to the stop button control circuit. If the may be any program file from 3 through 999. Entering
normally closed wiring configuration is used, the input a 0 in the time-base word disables the selectable timed
point receives power continuously unless the stop func- interrupt.
tion is desired. Any faults occurring with the stop circuit Programming the selectable timed interrupt is done
wiring, or a loss of circuit power, would effectively be when a section of program needs to be executed on a
equivalent to an intentional stop. time basis rather than on an event basis. For example, a

Inputs Ladder logic program Output
L1 Stop Start M L2

OL
Stop M M
M

Start

Figure 9-25 Wiring of stop buttons.

200 Chapter 9 Program Control Instructions

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