Service Peripherals and Force I/O
After the CPU reads the inputs from the input modules, it reads any attached peripheral
devices. This is primarily a communications service for any attached devices. For example, it
would read a programming device to see if any input, output, or other memory type status
needs to be modified. There are two basic types of forcing available with the DL06 CPUs:
• Forcing from a peripheral – not a permanent force, good only for one scan
• Bit Override – holds the I/O point (or other bit) in the current state. Valid bits are X, Y, C, T, CT,
and S. (These memory types are discussed in more detail later in this chapter).
Regular Forcing — This type of forcing can temporarily change the status of a discrete bit.
For example, you may want to force an input on, even though it is really off. This allows you
to change the point status that was stored in the image register. This value will be valid until
the image register location is written to during the next scan. This is primarily useful during
testing situations when you need to force a bit on to trigger another event.
Bit Override — Bit override can be enabled on a point-by-point basis by using AUX 59 from
the Handheld Programmer or, by a menu option from within DirectSOFT32™. Bit
override basically disables any changes to the discrete point by the CPU. For example, if you
enable bit override for X1, and X1 is off at the time, then the CPU will not change the state
of X1. This means that even if X1 comes on, the CPU will not acknowledge the change. So, if
you used X1 in the program, it would always be evaluated as “off ” in this case. Of course, if
X1 was on when the bit override was enabled, then X1 would always be evaluated as “on”.
There is an advantage available when you use the bit override feature. The regular forcing is
not disabled because the bit override is enabled. For example, if you enabled the Bit Override
for Y0 and it was off at the time, then the CPU would not change the state of Y0. However,
you can still use a programming device to change the status. Now, if you use the
programming device to force Y0 on, it will remain on and the CPU will not change the state
of Y0. If you then force Y0 off, the CPU will maintain Y0 as off. The CPU will never update
the point with the results from the application program or from the I/O update until the bit
override is removed. The following diagram shows a brief overview of the bit override feature.
Notice the CPU does not update the Image Register when bit override is enabled.
