Rockwell Automation 1761 MicroLogix 1000 Programmable Controllers - Manual (Page 172)

Preface MicroLogix 1000 Programmable Controllers User Manual
9–24
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Control is the address of the control structure. The control structure stores the
status bits, the stack length, and the position value. Do not use the control file
address for any other instruction.
Status bits of the control structure are addressed by mnemonic. These include:
– Empty Bit EM (bit 12) is set by the controller to indicate the stack is
empty.
– Done Bit DN (bit 13) is set by the controller to indicate the stack is full.
This inhibits loading the stack.
– FFU/LFU Enable Bit EU (bit 14) is set on a false-to-true transition of the
FFU/LFU rung and is reset on a true-to-false transition.
– FFL/LFL Enable Bit EN (bit 15) is set on a false-to-true transition of the
FFL/LFL rung and is reset on a true-to-false transition.
Effects on Index Register S:24
The value present in S:24 is overwritten with the position value when a false-to-true
transition of the FFL/FFU or LFL/LFU rung occurs. For the FFL/LFL, the position
value determined at instruction entry is placed in S:24. For the FFU/LFU, the
position value determined at instruction exit is placed in S:24.
When the DN bit is set, a false-to-true transition of the FFL/LFL rung does not
change the position value or the index register value. When the EM bit is set, a
false-to-true transition of the FFU/LFU rung does not change the position value or
the index register value.