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Old March 1st, 2018, 02:40 PM   #1
chutnik
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Question sqo instruction

how does an sqo instruction work with no bit to trigger the next step
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Old March 1st, 2018, 03:19 PM   #2
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Welcome to the forum!

The answer to that question may depend on the processor you're using. Tell us make/model/etc. of your PLC for starters.

If you have a screen shot of the code in question, posting that may help us as well.
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Old March 1st, 2018, 03:42 PM   #3
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using a allen bradley 1769-l33
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Old March 1st, 2018, 03:53 PM   #4
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Some other logic is driving the control.pos value. What you have there is a glorified MOV with indirect addressing (since the mask is all 1s).

Do a cross reference on TANK_QUEUE_SEQUENCER and/or TANK_QUEUE_SEQUENCER.POS
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Old March 1st, 2018, 04:19 PM   #5
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Or, it could be the .EN is being OTU'd elsewhere.

This will cause the next scan of the SQO to advance to the next step.

EDIT Mickey suggested this, but deleted his post....
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Last edited by daba; March 1st, 2018 at 04:21 PM.
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Old March 1st, 2018, 08:14 PM   #6
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Quote:
Originally Posted by daba View Post
Or, it could be the .EN is being OTU'd elsewhere.

This will cause the next scan of the SQO to advance to the next step.

EDIT Mickey suggested this, but deleted his post....
Yes. I am not a RSLogix5000 user.So deleted the post.

Reading the instruction help lead me to believe the instruction executes when it is enabled. Not from a false to true transition( like other platforms RSLogix500 and RSLogix5). If that is true then maybe the instruction is in a conditional subroutine.

RSLogix5000 users, please correct me.
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Last edited by Mickey; March 1st, 2018 at 08:28 PM.
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Old March 2nd, 2018, 12:12 AM   #7
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I believe that sometimes this instruction can be paired with an SQI instruction which is looking for the appropriate conditions to advance to the next step (or position using the instruction parameter)
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Old March 2nd, 2018, 07:54 AM   #8
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Quote:
Originally Posted by bernie_carlton View Post
I believe that sometimes this instruction can be paired with an SQI instruction which is looking for the appropriate conditions to advance to the next step (or position using the instruction parameter)
Irrespective of whether an SQO is paired with an SQI, each of them will require a false-to-true transition to make it advance to the next "index".

This is actually the case for nearly all instructions that have "Control" bits associated with them.

The control bits are used (amongst other things), to remember the last state of the rung, in most cases that is an "Enable" (.EN) bit, to determine if the rung has gone false-to-true.

Counters have 2 "Enable" bits, .CU (Count Up Enable), and .CD (Count Down Enable).

Remember that, in the PLC world, each and every instruction only knows one thing, the current state of the rung, true or false, when it is executed. They have absolutely no knowledge of what precedes them, or follows them, they just receive the "Rung Logic Continuity" as an input parameter (the "Enable_In"), and acts on the state of that accordingly. The "Enable_In" state is passed on to the next instruction as the "Enable_Out", mirroring the input state, so that following instructions can see the Rung Logic Continuity, true or false.

For this reason we can always fool an instruction into thinking the rung has changed state false-to-true, by resetting its "Enable_Out" bit anywhere else in the logic. The next scan of the instruction, assuming Enable_In is true, determines that since "Enable_Out" is false, the rung will appear to have changed state, false to true, even though it actually hasn't.

If a programmer wants a "controlled" instruction to execute every time it is scanned, it is quite common to see an OTU of the instruction's "Enable_Out" either in front of, or behind, the instruction.

Timers are an exception to this false-to-true "rule", timers being "Enabled" do not stop the instruction working each and every scan, for obvious reasons.
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