Read Inputs; Service Peripherals And Force I/O - AutomationDirect C0-USER-M Manual

Read Inputs

The CLICK PLC reads the status of all inputs, then stores it in the image register. Input image
register locations are designated with an X followed by a memory location. Image register data
is used by the PLC when it solves the application program.
Of course, an input may change after the PLC has just read the inputs. Generally, the PLC
scan time is measured in milliseconds. If you have an application that cannot wait until the
next I/O update, you can use Immediate Instructions. These do not use the status of the input
image register to solve the application program. The Immediate instructions immediately read
the input status directly from the I/O modules. However, this lengthens the program scan
since the PLC has to read the I/O point status again.

Service Peripherals and Force I/O

After the CLICK PLC 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 CLICK PLC:
• 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 nd CT. (These memory types are discussed in more detail earlier in this chapter).
Forcing and Bit Override are done through the Data View Monitor.
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: This is a more forceful type of bit manipulation. When bit override is enabled,
you can actually override the current status of a bit in the image register. This change will
remain intact until you remove the override.
CLICK PLC Hardware User Manual, 6th Edition, Rev. F – C0-USER-M
Chapter 2: Specifications
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