Introduction to Programming
Chapter 4
43
Programmable controllers have many of the capabilities of hardwired relay
control systems. Control functions similar to those available with relays are
provided by the following relay-type instructions:
Examine instructions
Output instructions
Branch instructions
There are two examine instructions:
Examine On –| |–
Examine Off –| / |–
They command the processor to check the on/off status of a specific bit
address in memory. A one or zero stored at the bit address may represent
the actual on or off status of a single input or output device.
Examine instructions ar programmed in the condition area of the ladder
diagram rung (Figure 4.3). As condition instructions, their on or off states
determine the true or false condition of the rung. Any bit in the data table,
excluding the processor work areas, can be addressed by an examine
instruction. A single bit can be examined several times within the same
rung or program.
Figure 4.3
Areas
of the Ladder Diagram Rung
|| ()|| || ||
|| || ||
Output
Instruction
Condition
Instructions
A Continuous Path
is Needed for
Logic Continuity
Examine On Instruction
The Examine On instruction tells the processor to check the status of the
addressed memory bit for an on (one) condition. When addressing the I/O
image table, this instruction can examine a single input or output bit for an
on voltage state.
4.3
RelayType Instructions
4.3.1
Examine Instructions
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