Rockwell Automation T8082 Trusted Toolset Suite - Manual (Page 68)

Trusted
TM
Toolset Suite T8082
Issue 14 Feb 10 PD-T8082 68
6.4. I/O Forcing
As part of the operation and maintenance of a safety system, there are situations that arise requiring
the forcing (locking) of data points. It is always an advantage to be aware of the fact that data points on
the system have been forced. The toolset now provides extensive monitoring facilities of forced data,
as shown in Figure 51 below.
Figure 51 Forced Data Point
When a data point is forced, a symbol is displayed in the logic and dictionary to reflect that the point
has been forced. In the dictionary, both the forced value and the true field value are displayed as
illustrated in Figure 51 above. The actual field value on an I/O point is contained within brackets. In the
example above, the field value is –195 and the forced value is 2000.
The ISaGRAF Users’ Guide section A.26.2 describes the way forcing operates. Forcing removes the
connection between the application variable and the physical module. Forcing is not allowed on internal
variables.
For an input, the variable is disconnected from the module. The forced value is applied to the variable,
so that the application sees the forced value. The real input is still visible in the dictionary, shown in
brackets above.
For an output, the variable is also disconnected from the module. In this case, the application is unable
to change the real output. However, forced values are applied to both the application variable and to
the real output. Normally the application will immediately overwrite this value, but if the output is read
before it is written, the forced value will be read until the point in the code where the output is written.
Therefore it is not recommended to read from an output variable before it is written, e.g. in creating a
latching ladder rung.
SOE is disabled for forced I/O points.