ANNUNCIATORS (F BITS) VS NORMAL (M BITS) FOR ALARMS

[netdomain 0]

Hi , What is a difference between using F bits for alarms vs M bits for alarms? Any advantages to using F bits?

[Crossbow 297]

F bits are internal devices, just like M bits. One difference is that the number of the active bit can be found in a data register. So if F12 is on, the number 12 is in a special D register (which one escapes me at the moment). On the Q Series, the F bits can also turn on the USER LED on the front of the CPU.

[The Cat 0]

In addition to this, the F coil reference address (often referred to as annunciators) is transferred to a stack (at least in FX) to I think a maximum depth of 8. Dedicated D8*** registers hold the most recent F coil address (top of stack) and the number of filled positions in the stack. New F addresses are pushed on and the stored addresses can be popped off under program control making a useful error detection or event record. The number of filled stack positions is automatically adjusted. Stack depth, etc may vary for other models.