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Chapter 3
These memory locations cannot be accessed by the user. Their word
addresses are not available for addressing of any kind. The processor uses
both areas for internal control functions.
Output Image Table
The primary function of the output image table is to control the status of
outputs wired to the output modules. If the output image table bit is on, its
corresponding output is on. If the bit is cleared to off, its corresponding
output is off. These bits are controlled by instructions in the user program.
The processor controls the status of bits in the output image table as it
generates output commands. Actual hardware outputs change state only if
corresponding output image table bits change state or if they are forced.
NOTE: PLC-2/30 output terminals can be forced on or off through the
industrial terminal. The output image table bits, corresponding to output
terminals which are forced, do not change state.
The output image table ordinarily begins with word 010
and ordinarily
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ends with word 027
. However, word 027 is reserved and output or block
8
transfer modules must not be placed in rack 2, I/O group 7.
The output image table therefore contains 16 word addresses, or 256
bit addresses. Using the industrial terminal, the output image table can
be reduced to 8 word addresses (128 bit addresses), or increased from
16 word addresses to 56 (896 bit addresses). By changing memory
configuration to 896 I/O (seven 1771-A4B I/O chassis with 2-slot
addressing), the 896 bit addresses represent the maximum number of
discrete outputs the processor can control.
Each bit in the output image table may be associated with a hardware
terminal address, although this is not always the case, since a
corresponding output module may not actually be placed in this I/O rack
slot. If it is, however, the terminal address is the same as the bit address.
A secondary function of the output image table is to provide a storage area
for bits or words. Words and/or bits in the output image table not actually
used to store the on/off status of devices can be used for data storage.
NOTE: Although only 11 bits of word 027
are actually used as processor
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control bits, the remaining bits must not be used since inadvertent
alteration of these bits could occur. The processor sets bit address 027/00
8
ON and OFF to indicate a low battery condition. This bit can be examined
by instructions in the user program. (See Section 2.4)
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