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5–8
Single Transfer Programming
Multiplexing
Publication 1771 UM006B-EN-P - June 2002
Rung 4 shows the input status word examined in the user program.
Note that this word is automatically in the input image table when
single transfer is selected. The processor automatically updates this
word each I/O scan.
In summary, when you are not using preset words, you need only set
bits of the output image table word which corresponds to the module.
This word then serves as the output control word. However, when
using one or both preset words, the output image table word is used
for multiple purposes and intermediate storage word addresses must
be used for output control and preset word storage. The following
sections outline methods for multiplexing the output words in such
applications.
The processor writes a single word - the output image table word - to
the encoder/counter module each I/O scan. Multiplexing is a
programming technique by which the processor can share this single
output image table word and use it to send multiple words to the
module, one word at a time. Multiplexing, therefore, is useful when
one or more preset words are needed.
Multiplexing requires an orderly sequencing of events in the
program. The program must coordinate control of the output image
table word such Multiplexing requires an orderly sequencing of
events in the program. The program must coordinate control of the
output image table work such that it sends the intended output word,
whether control or preset, to the module. In this publication, a scan
counter is used for this coordination. While it is not the only method
of control for multiplexing, the scan counter is a direct and easily
understood method and can be used with any PLC or PLC-2 family
processor.
To understand the scan counter's function, consider how information
must be transferred to the module. In one program scan, the
processor stores the word that is to be sent to the module in the
output image table word for the module. During the very next I/O
scan, the processor automatically outputs the image table word to the
module.
Table 5.A lists a sequence of operation for single transfer in a typical
application, showing what must happen at each value of the scan
counter. This example assumes that both preset words are used. If
only one of the preset words is needed, only counts 1 and 2 of the
scan counter are necessary.
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