Omron SYSMAC C20P Operation Manual page 25

IR Area
Output Bit Usage
Word Allocations
16
tions as required to achieve effective and proper control. They cannot be
used in instructions that control bit status, e.g., the Output, Differentiation Up,
and Keep instructions.
Output bits are used to output program execution results and can be used in
any order in programming. Because outputs are refreshed only once during
each scan (i.e., once each time the program is executed), any output bit can
be used only one instruction that controls its status, including OUT,
KEEP(11), DIFU(13), DIFD(14) and SFT(10). If an output bit is used in more
than one such instruction, only the status determined by the last one will ac-
tually be output from the PC.
As outputs are refreshed only once during each scan (i.e. once each time the
program is executed), any output bit can be used in only one instruction that
controls its status, including OUT, OUT NOT, KEEP(11), DIFU(13), DIFD(14),
and SFT(10). If an output bit is used in more than one such instruction, only
the status determined by the last instruction will actually be output from the
PC. See 5–12–1 Shift Register – SFT(10) for an exception to this rule.
The maximum number of words available for I/O within the IR area is 10,
numbered 00 through 09. The remaining words (10 through 18) are to be
used for work bits. (Note that with word 18, only the bits 00 through 07 are
available for work bits although some of the remaining bits are required for
special purposes when RDM(98) is used).
The actual number of bits that can be used as I/O bits is determined by the
model of the CPU and the PC configuration. There are different models of
Expansion I/O Units and Special I/O Units and I/O Link Units which can be
connected to any of the CPUs. Each CPU model provides a particular num-
ber of I/O bits and each Expansion I/O Unit, Special I/O Unit or I/O Link Unit
provides a particular number of I/O bits. Configuration charts for the possible
combinations of CPUs and Units are included later in this section. Refer to
those to determine the actual available I/O bits.
With P-type PCs, IR 00 through IR 04 are always input bits and IR 05 through
IR 09 are always outputs bits. These are allocated in order from IR 00 (input)
and IR 05 (output) beginning from the CPU. Each Unit is allocated either one
input word and one output word or, for the C40P/C60P Units, two input words
and two output words. If the words or bits within a word are not need by the
Unit, they are not allocated to any other Unit. Unallocated input bits cannot
be used for any purpose, but unallocated output bits can be used in program-
ming as work bits.
Section 3–3