Introduction
A contact is an LD element that transfers a status on the horizontal link to its right side. This status is the result of a Boolean AND operation on the status of the horizontal link on the left side with the status of the relevant Boolean actual parameter.
A contact does not change the value of the relevant actual parameter.
Contacts take up one cell.
The following are permitted as actual parameters:
Boolean variables
Boolean constants
Boolean addresses (topological addresses or symbolic addresses)
ST expression delivering a Boolean result (e.g.
VarA OR VarB)ST expressions as actual parameters for contacts are a supplement to IEC 61131-3 and must be enabled explicitly
Contact Types
The following contacts are available:
Designation |
Representation |
Description |
|---|---|---|
Normally open |
|
In the case of normally open contacts, the status of the left link is transferred to the right link if the status of the relevant Boolean actual parameter (indicated with xxx) is ON. Otherwise, the status of the right link is OFF. |
Normally closed |
|
In the case of normally closed contacts, the status of the left link is transferred to the right link if the status of the relevant Boolean actual parameter (indicated with xxx) is OFF. Otherwise, the status of the right link is OFF. |
Contact for detecting positive transitions |
|
With contacts for detection of positive transitions, the right link for a program cycle is ON if a transfer of the relevant actual parameter (labeled by xxx) goes from OFF to ON and the status of the left link is ON at the same time. Otherwise, the status of the right link is 0. Also see Edge Recognition. |
Contact for detecting negative transitions |
|
With contacts for detection of negative transitions, the right link for a program cycle is ON if a transfer of the relevant actual parameter (labeled by xxx) goes from ON to OFF and the status of the left link is ON at the same time. Otherwise, the status of the right link is 0. Also see Edge Recognition. |
