S5 AWL Command: U (AND) / UND

The U instruction performs a logical AND operation on the VKE (Verknüpfungsergebnis / Result of Logic Operation). It reads a boolean operand and ANDs it with the current VKE.

Syntax#

U  <operand>

Valid operands: E (Input), A (Output), M (Marker/Flag), T (Timer), Z (Counter), D (Data bit)

S7 Equivalent#

In S7 STL (English mnemonics), U becomes A (AND). In German mnemonics, U remains U.

Code Example#

S5 AWL — Two inputs AND-linked to one output:

U  E 0.0       // Input 0.0 (e.g., Start button)
U  E 0.1       // Input 0.1 (e.g., Safety door closed)
=  A 4.0       // Output 4.0 (e.g., Motor on)

Equivalent S7 SCL:

Motor_On := Start_Button AND Safety_Door_Closed;

Behavior: A 4.0 is TRUE only when both E 0.0 AND E 0.1 are TRUE.

VKE Behavior#

• First U in a logic chain: VKE = value of the operand
• Subsequent U: VKE = VKE AND operand
• After =, S, R: VKE is reset (new chain begins)

Common Errors#

Forgetting that U starts a new VKE chain after = / S / R:

U  E 0.0
=  A 4.0       // VKE reset here
U  E 0.1       // This starts a NEW chain (not AND with E 0.0)
=  A 4.1

Confusing U with O: U = AND (both must be true), O = OR (either can be true).

Related Commands#

• UN (AND NOT) — AND with negation
• O (OR) — OR operation
• = (Assign) — Assign VKE to output

Part of the S5 AWL Command Reference. Maintained by PLCcheck.ai.

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