AB SLC 100: Sequencing Logic Is the Fix, Not Hardware

On the panel, 011 either never starts the ten-second cycle, pauses in the wrong state, or starts again immediately after cycle 500. Start here: this is a sequencing and reset problem built around latch 701, retentive timers 901/902, and counter 903. That is not a hardware fault.

Stop applying the wrong fixes

Trace the actual sequence

Use 701 as the run-state latch. The start contact sets it, and counter 903 unlatches it after 500 completed cycles.

• 901 measures the output-on interval with a preset of 10.0 seconds.
• The done state of 901 starts 902, which measures the output-off interval with the same preset.
• 011 is energized while 701 is set and 901 is not done.
• The completion of 902 increments 903 once, then resets both timers for the next cycle.
• The normally closed 002 condition inhibits both timer rungs. Because the timers use RTO, their accumulated values remain available for the resume.

The source notation identifies the physical controls as 001 or 101 for start and 002 or 102 for pause. Use the address format accepted by the installed controller and programmer; do not mix both formats in one program.

Enter the timer-counter logic

1. Latch 701 from the start input.
2. Run 901 while 701 is active and pause input 002 is off.
3. After 901 completes, run 902 under the same pause condition.
4. Count the completion of 902 with 903.
5. Reset the timers at the end of each complete on/off cycle.
6. Unlatch 701 when 903 reaches 500.

Rung 1  -| |- 001 --------------------(L)- 701
Rung 2  -|/|- 002 -| |- 701 ----------(RTO)- 901  PRE = 10.0
Rung 3  -|/|- 002 -| |- 701 -| |- 901 -(RTO)- 902  PRE = 10.0
Rung 4  -| |- 902 --------------------(CTU)- 903  PRE = 500
Rung 5  -| |- 902 --------------------(RST)- 901
Rung 6  -| |- 902 --------------------(RST)- 902
Rung 7  -| |- 903 --------------------(U)- 701
Rung 8  -| |- 903 --------------------(RST)- 903
Rung 9  -| |- 701 -|/|- 901 ----------( )- 011

Read contacts 901, 902, and 903 here as their completion states. Confirm the exact entry and monitoring syntax on the 1745-PT1 before placing the sequence in service.

Make the pause action intentional

Closing pause input 002 makes its normally closed contacts false. Both timer rungs stop executing, but 701 stays latched and the retentive timer keeps its accumulated value.

• If you pause during the on interval, 901 stops accumulating and 011 remains energized because rung 9 does not contain 002.
• If you pause during the off interval, 901 remains done, 902 stops accumulating, and 011 remains off.
• The counter does not advance until 902 completes, so a partial interval is not recorded as a cycle.

This is a hold-state pause, not a force-output-off pause. If the load must de-energize whenever pause is pressed, add the pause condition to the output logic—but recognize that this changes the requested timing sequence. Decide which behavior the process requires before editing rung 9.

Verify every transition

1. Release the start input after initiating the test. Confirm that 701 latches.
2. Verify that 011 energizes immediately and stays on while 901 accumulates toward 10.0.
3. At the first timer completion, verify that 011 turns off and 902 begins accumulating.
4. Operate pause once during each interval. Confirm that the active timer stops at its current value and resumes from that value when pause opens.
5. At the completion of 902, verify one increment of 903 and the reset of both timers.
6. Confirm that the next cycle begins with 011 on and that the counter does not increment on either the start or pause transition.
7. Test the terminal count with the start input released. When 903 reaches its preset of 500, verify that 701 unlatches and 011 remains off.

For a controlled bench test, temporarily use shorter presets or a lower terminal count, then restore 10.0, 10.0, and 500. Recheck the final values from the programmer before connecting the process load.

Prevent restart and display traps

Rungs 7 and 8 stop the run and immediately reset 903. If input 001 remains closed, rung 1 can latch 701 again on the next scan. Use a momentary start, require the operator to release start, or add a one-shot/start-inhibit arrangement supported by the installed controller.

Automatic counter reset also means the final value of 500 will not remain available for observation. If the terminal count must stay visible on the 1745-PT1, remove the automatic counter reset from the terminal event and reset 903 from a separate clear or new-batch command. Verify first that the handheld programmer can display the counter value in its monitor mode.

Test power interruption behavior separately. An RTO instruction provides retentive timing when its rung goes false, but the required restart policy also depends on the retained state of the run latch, timers, counter, and controller startup processing.

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