22 Publication 1756-PM004C-EN-P - October 2009
Chapter 1 Communicate with I/O Modules
Buffer I/O
Buffering is a technique that logic does not directly reference or manipulate
the tags of real I/O devices. Instead, the logic uses a copy of the I/O data.
Buffer I/O in the following situations:
• To prevent an input or output value from changing during the execution
of a program. (I/O updates asynchronous to the execution of logic.)
• To copy an input or output tag to a member of a structure or element of
an array.
Follow these steps to buffer I/O.
1. On the rung before the logic for the function, copy or move the data
from the required input tags to their corresponding buffer tags.
2. In the logic of the function, reference the buffer tags.
3. On the rung after the function, copy the data from the buffer tags to the
corresponding output tags.
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