How To Force IO In Ladder Logic (RSLogix 5000)

Force Bar

Forcing IO is one of the handiest ways of troubleshooting logic and checking peripheral devices.

Before you start forcing things lets look at some of the “Dos and don’ts”  when forcing IO.

First, forces are enabled, disabled and removed from the online toolbar. Take a look at the picture above, do you see the area that says “Forces”, this is the online toolbar.Force Selection

Forces cannot be addressed to a single module, tag group or peripheral device. Forces need to be enabled to take effect.  Forcing tags is done directly from ladder logic or from the tag monitor window by highlighting the tag, right clicking and selecting the appropriate action from the list:

Force On
Force Off
Remove Force
 

The front of the controller has an I/O Force status indicator which displays one of the following three statuses:

Off – No forces installed and I/O forces are disabled.
 
Flashing – At least one tag contains a force value and I/O forces are disabled.
 
Solid – I/O forces are active and force values may or may not exist.
 

Forcing an input on or off may not yield the desired effect.Take a look at the following example.

The “MOTOR_RUN” output is forced on. The “MOTOR_RUN” output is on; however in the very next rung the XIC instruction referencing the MOTOR_RUN tag is false. The memory associated with the MOTOR_RUN address is not changed from a force, but rather the bits Force Mask.Force On Output

As you can see in the picture below the value of MOTOR_RUN is 0, but the Force Mask is set to 1. The ladder logic looks at the data in the Value column not the Force Mask column when executing the ladder program.

Force Monitor

To summarize, in order to force a bit on or off follow these steps:

Force On & Off

  1. Enable forces from the online toolbar
  2. Highlight the bit you would like to force and right click
  3. Select Force On or Off from the pop-up window

Enable all Forces or Disable all Forces

  1. Locate the Forces area of the Online Toolbar
  2. Click the Forces button and select Enable all IO forces or Disable all IO forces