Page 216 - Programmable Logic Controllers, Fifth Edition - Mobile version

Data File I1 (bin) . . INPUT Forces
O set 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
I:1.0 . . . . . . . . . . . . 1 . . .
. . . .
I:2.0

I:1.0/3 Radix:
Symbol: Columns:
Desc:
Enable Remove All Data File Help

Figure 9-20 Forces version of the data table with bit I:1/3
forced on.

2. With the right mouse button, click the I/O bit you
want to force.
3. From the menu that appears, select Go to Data Table
or select Force On or Force Off.
4. From the associated data table that appears, click on
the Forces button. Figure 9-21 Exercise care when you use forcing functions.
5. The Forces version of the data table appears with Source: Courtesy Givens Engineering Inc.
the selected bit highlighted. Click on this bit
with the right mouse button. 9.7 Safety Circuitry
6. From the menu that appears, you can force the
selected bit on or off. Sufficient emergency circuits must be provided to stop
either partially or totally the operation of the controller or
Exercise care when you use forcing functions. If the controlled machine or process. These circuits should
used incorrectly, force functions can cause injuries be hardwired outside the controller so that in the event
to persons working around a system, and/or equip- of total controller failure, independent and rapid shut-
ment damage. For this reason, forcing functions should down is available.
be used only by personnel who completely understand Figure 9-22 shows typical safety wiring requirements
the circuit and the process machinery or driven equip- for a PLC installation. The safety requirements of this
ment (Figure 9-21). You must understand the potential installation can be summarized as follows:
effect that forcing given inputs or outputs will have on
machine operation in order to avoid possible personal • A main disconnect switch is installed on the incom-
injury and equipment damage. Before using a force ing power lines as a means of removing power from
function, check whether the force acts on the I/O point the entire programmable controller system.
only or whether it acts on the user logic as well as on the • The main power disconnect switch should be
I/O point. Most programming terminals and PLC CPUs located where operators and maintenance personnel
provide some visible means of alerting the user that a have quick and easy access to it. Ideally, the discon-
force is in effect. nect switch is mounted on the outside of the PLC
In situations in which rotating equipment is involved, enclosure so that it can be accessed without opening
the force instruction can be extremely dangerous. For the enclosure.
example, if maintenance personnel are performing routine • In addition to disconnecting electrical power, you
maintenance on a de-energized motor, the machine may should de-energize, lock out, and tag all other
suddenly become energized by someone forcing the motor sources of power (pneumatic and hydraulic) before
to turn on. This is why a hardwired master control circuit you work on a machine or process controlled by the
is required for the I/O rack. The hardwired circuit will pro- controller.
vide a method of physically removing power to the I/O • An isolation transformer is used to isolate the con-
system, thereby ensuring that it is impossible to energize troller from the main power distribution system and
any inputs or outputs when the master control is off. step the voltage down to 120 VAC.

Program Control Instructions Chapter 9 197

pet73842_ch09_184-206.indd 197 03/11/15 4:01 PM