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====== MES-Terminal interlocks ======
Three Boolean variables are required.

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|^Variable	^Type	^Description	^Direction|
|HeartbeatToMES	|BOOL	|Blinker signal from PLC in three-second cycle (maximum 3 sec. signal pause and maximum 3 sec. signal)	|From PLC to MES|
|HeartbeatToPLC	|BOOL	|Blinker signal from MES to PLC 	|From MES to PLC|
|EnableMachine	|BOOL	|MES set this variable. The machine may only start automatic mode if this variable is set to TRUE. **Attention**: If the status of HeartbeatAnSPS does not change for e.g. ten seconds, EnableMachine has to be ignored.	|From MES to PLC|

===== Signal status diagram 'MES terminal interlocks' =====
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The pulse can be generated either by the PLC internally or in TC2/3 by the block MES_Lifebit.

  * Section 1: Machine gets release from MES-Terminal.
  * Section 2: MES-Terminal withdraws machine's enable (the next cycle should not be able to start). Recommendation: If EnableMachine = FALSE and the next cycle should be started, there should be a PLC error message, e.g. "Machine has no release from MES-Terminal".
  * Section 3: Use case "Production without MES system": If HeartbeatToPLC is not written, the machine is released as failover.

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|^HeartbeatToMES ^HeartbeatToPLC	^EnableMachine ^Is machine enabled?|
|Toggle	|Toggle	|TRUE |TRUE (see section 1)|
|Toggle	|Toggle	|FALSE |FALSE (see section 2)|
|Toggle		|FALSE |TRUE |FALSE (not possible)|
|Toggle		|FALSE 	|FALSE |TRUE (see section 3)|

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