Custom Tag Initialization During Prescan

Only safety tags that are configured as constant value tags are captured as part of the safety signature.

Give special consideration to instructions that use pseudo-operands, such as the following:

Unless modified by the application, pseudo-operands are initialized only once when the application is downloaded. For details, see the 'Pseudo-operand Initialization' online Help topic. Before the controller is in Run mode, you must initialize the .PRE and .LEN values for the preceding instruction tags and other non-constant safety tags that are used in a safety critical operation. Initialize these values by using one of these methods:

For more information about how to perform a custom tag initialization during prescan, see the Logix 5000 Controllers Design Considerations Reference Manual, publication 1756-RM094.

You can use the SaveSnapshot routine to copy non-constant safety tag values to the safetyPrescanInitUDT backup, which consists of safety tag types, such as CTU preset, FAL length, TON preset, DINT array, REAL, and BOOL. For example, once the application is downloaded and configured, toggle the saveSnapshot tag to call the SaveSnapshot routine to initialize safetyPrescanInitUDT. Upon subsequent transitions to RUN mode, the prescan routine of safetyPrescanInitAOI reinitializes the non-constant safety tag values from the safetyPrescanInitUDT backup.