Page 215 - Programmable Logic Controllers, Fifth Edition - Mobile version

Output module
Status of bit
O:2/5 remains
Output image table at 0
ON
O:2/5

OFF
0 0
O:2/6
M
Field output
devices

Ladder
Remote Run Forces Installed logic
Forces Enabled program

Input Ladder logic program Outputs
L1 I:1/3 O:2/5 L2
I:1/3
O:2/5
OFF Force> ON
ON
O:2/5 O:2/6 O:2/6 M

OFF
Force> ON
Figure 9-19 Forcing an output on.

at the address is usually not affected. Figure 9-19 illustrates hardwire jumpers. With PLC control, hardwire jump-
how an output is forced on. The operation of the program ers are not necessary because the input data table
can be summarized as follows: values can be forced to an on or off state. The force
function allows you to override the actual status of
• The processor ignores the actual state of solenoid
output O:2/5. external input circuits by forcing external data bits on
or off. Similarly, you can override the processor logic
• The programming device sets the force state in the and status of output data file bits by forcing output bits
output force data file and the PLC implements the on or off. By forcing outputs off, you can prevent the
force to turn solenoid output O:2/5 on even though controller from energizing those outputs even though
the output image table file indicates that the user the ladder logic, which normally controls them, may
logic is setting the point to off. be true. In other instances, outputs may be forced on
• M output O:2/6 remains off because the status even though logic for the rungs controlling those out-
bit of output O:2/5 is not affected by the force puts may be false.
instruction. Figure 9-20 shows the forces version of the data table
• Not all brands of PLCs operate this way. For ex- with bit I:1/3 forced on. You can enter and enable or dis-
ample, forcing an output with a GE Fanuc controller able forces while you are monitoring your file offl ine, or
will cause the contacts that have the same address as in any processor mode while monitoring your file online.
the output to also change to the appropriate state. With RSLogix 500 software, the steps are as follows:
Overriding of physical inputs on conventional relay 1. Open the program file in which you want to force
control systems can be accomplished by installing the logic on or off.

196 Chapter 9 Program Control Instructions

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