CPU Specifications
and Operation
4--13
CPU Specifications and Operation
DL105 PLC User Manual, 3rd Edition
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.
Forced I/O— temporarily changes 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 just need to force a bit on to trigger another event.
Forced Inputs — The CPU reads the status of X inputs during the Read Inputs
portion of the scan. When the CPU services the programming device, it logs any
request to force an X input on. If the input is used in the application program, the
ladder X contact is considered closed (on). Since an X input is a real-world input
point, the CPU will change the s tatus when it reads the inputs on the next scan.
Forced Outputs— Outputs which are not used in the program can be forced on and
off for troubleshooting and maintenance purposes. You can temporarily allow the
forcing of any output by inserting an END coil instruction at the beginning of the
ladder program. Then you can use DirectSOFT or a HPP to force outputs on and off.
The DL105 PLCs only retain the forced value for one scan. There is an exception to
this rule. For example, if the point address is greater than X11 or Y7 or it is not used in
the ladder program, then the point will maintain the forced status.
WARNING: Only authorized personnel fully familiar with all aspects of the
application should make changes to the program. Make sure you thoroughly
consider the impact of any changes to minimize the risk of personal injury or
damage to equipment.
There are certain V-memory locations that contain Special Relays and other
dedicated register information. This portion of the execution cycle makes sure these
locations get updated on every scan. Also, there are several different Special
Relays, such as diagnostic relays, etc., that are also updated during this segment.
Read Inputs
Service Peripherals
and Force I/O
Update Special
Relays and Special
Registers
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