Allen-Bradley 1756-RM

Allen-Bradley 1756-RM  Enhanced Redundancy System

Product information：

Brand：Allen-Bradley                              Model number：1756-RM

Condition：new                              Warranty: 12 month

Lead Time：3-day working da        Country of origin: United States

Price: Please contact us                   Product weight：0.225kg

HS: 8517623990

Product technial information：

CORRECTED: An anomaly can occur when you nest simultaneous branches in a sequential function chart (SFC) routine as shown in 

this graphic.

During normal operations, the logic does not step out of a simultaneous branch until all incoming legs have reached their 

bottom-most step. In the example that is shown, the logic does not step out of the Outer simultaneous branch until the left-most leg 

has stepped out of the Inner simultaneous branch.

The first time through the SFC routine, the application works as expected. On subsequent scans, however, the chart steps out of the 

Outer simultaneous branch even though the left leg has not reached the Inner simultaneous branch yet. 

This behavior occurs because the information in the bottom step, that is, the step next to the word Outer, remains from the previous 

scan. This stale information incorrectly causes the transition to move on as if the left leg is at the bottom step when it actually is not.

To work around this anomaly, confirm that all incoming legs have reached their bottom steps in the transition. You can check the .x 

bits to make sure that the bottom-most step of each converging leg is active before allowing the chart to advance. This check can be 

added to the transition logic already present.

For example, the steps in the following expression are the bottom steps (see the previous graphic) of the incoming legs. Each step has 

a timer. The transition does not move on until all step timers have reached their preset value.

The expression can be modified to be sure that those steps are truly active. This expression is the original expression:

// Unit is ready to Transition

Step1_ready.dn and Step2_ready.dn

This expression is the modified expression to work around this anomaly.

// Unit is ready to Transition

Step1_ready.x and Step2_ready.x and Step3_ready.x and Step4_ready.x and Step5_ready.x and Step1_ready.dn and 

Step2_ready.dn and Step3_ready.dn and Step4_ready.dn and Step5_ready.dn

IMPORTANT: The x bit is cleared when the chart transitions out of a step.

If you add this check to the condition, it forces the engine to wait until all of those steps are genuinely active before advancing out of 

the simultaneous branch.

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