Difference Between Hard and Soft Bit Memory Transitionals

This article refers to G&L PiC application note document AN000002 and describes the difference between hard and soft bit memory transitionals.

​Hard bit memory will update the transitionals any time a write is done to the memory location for that BOOL variable (this is an automatic hardware function).  Soft bit memory updates transitionals when a coil is evaluated.

When using either the NETRCV function or COMM900, when an array of BOOLs is sent over ARCNET to the control, the transitionals will be set if hard bit memory is used, then cleared the next time either NETRCV, COMM900, or your .LDO writes to that memory location.  If soft bit memory is used, the transitionals will only be set and cleared when an evaluation of that coil occurs.  They will not be updated by the function alone. If the coil is never used in coil form in the .LDO, the transitional states for an array of BOOLs written to by NETRCV are not valid with soft bit memory.

A similar thing happens when COMM900 is used.  Since COMM900 updates the array of BOOLs using variable indexes, the transitionals will remain on until the next time the coil is used in your .LDO, or the next time the COMM900 function block writes to that coil, with either hard or soft bit memory.