Using memories

[Borzu]

To energize an output channel,why is it set an internal memory first when programming?

 

[DamianInRochester]

Try to rephrase your question differently. I am not certain I understand what you are asking.

If you are asking why a programmer might use internal bits to control the physical output bits there are a few reasons.

1) Helps with splitting up various modes (Auto/Manual) etc.
2) Allows logic and programs to be written prior to finalizing the IO.
3) Creates a single point of for which to locate the origin logic for that output.
4) Allows more control over when physical IO is actually written to. Can be help with controllers that support an asynchronous scan.

 

[bernie_carlton]

I will answer this from another perspective.

The communication channel from the CPU which is interpreting the program to the actual physical I/O is often many times slower than the access to a memory map which will represent the outputs.

Thus, during the program scan the intended outputs are written to the memory mapped section. At the end of the program scan the memory mapped area is sent to the actual physical outputs in one move, saving a lot of time.

Some instructions (if they exist in your system) like 'interrupt I/O read/writes' can override this sequence but at the expense of using precious program scan time.

So, from my point of view, the use of intermediate memory mapped locations which are then, at the end of the scan, applied to real outputs is to save time. The use of memory mapped inputs is another issue in itself.