Page 214 - Programmable Logic Controllers, Fifth Edition - Mobile version

9.6 Forcing External I/O Addresses
on. The operation of the program can be summarized as
follows:
The force function is essentially a manual override con-
trol function. Forcing allows the PLC user to turn an ex- • The processor ignores the actual state of input limit
ternal input or output on or off from the keyboard of the switch I:1/3.
programming device. This is accomplished regardless of • Although limit switch I:1/3 is off (0 or false) the
the actual state of the field device. The forcing capability processor considers it as being in the on (1 or true)
allows a machine or process to continue operation until a state.
faulty field device can be repaired. It is also valuable dur- • The program scan records this, and the program is
ing start-up and troubleshooting of a machine or process executed with this forced status.
to simulate the action of portions of the program that have • In other words, the program is executed as if the
not yet been implemented. limit switch were actually closed.
Forcing inputs manipulates the input image table file
bits and thus affects all areas of the program that use those Forcing outputs affects only the addressed output termi-
bits. The forcing of inputs is done just after the input scan. nal. Therefore, since the output image table file bits are un-
When we force an input address, we are forcing the sta- affected, your program will be unaffected. When we force
tus bit of the instruction at the I/O address to an on or an output address, we are forcing only the output terminal
off state. Figure 9-18 illustrates how an input is forced to an on or off state. The status bit of the output instruction

Input module
Actual state of input
OFF I:1/3 device (0) ignored

Field input Input image table
device

1

Remote Run Forces Installed Programming terminal
forces the state of
Forces Enabled Ladder
input I:1/3 ON (1) logic
program

Input Ladder logic program Outputs
L1 I:1/3 O:2/5 L2
I:1/3
OFF Force> ON O:2/5
ON

O:2/5 O:2/6
O:2/6 M
ON

Figure 9-18 Forcing an input on.

Program Control Instructions Chapter 9 195

pet73842_ch09_184-206.indd 195 03/11/15 4:01 PM