Automationdirect.com DL05, User Manual

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CPU Specifications
and Operation
4–13
CPU Specifications and Operation
The CPU 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 CPU when it solves the application program.
Of course, an input may change after the CPU has just read the inputs. Generally,
the CPU 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 CPU has to read the
I/O point status again. A complete list of the Immediate instructions is included in
Chapter 5.
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 DL05 CPUs.
S
Forcing from a peripheral – not a permanent force, good only for one
scan
S
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 Direct SOFT

.
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.
Read Inputs
Service Peripherals
and Force I/O