voyager2
Member
Excuse me for me simple question.. What is an First Scan bit? What can I use it for?
Voyager2
Voyager2
First Scan bit
- Thread starter voyager2
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TConnolly
Lifetime Supporting Member
Welcome to the forum.
A first scan bit it a bit that the PLC turns on during the first scan of a PLC program when the PLC first powers up or when it changes from program mode to run mode. The bit is true for only the first scan of the program.
The first scan bit is useful for initializing retentive memory variables and resetting states when it is necessary that the equipment begin operation in a specific condition.
In some PLCs that support multiple programs and tasks there may be multiple first scan bits for each task which is true for the first time the task scans its programs. This distinction matters if you have a periodic or event triggered task that executes much later than the other tasks in the PLC.
Some PLCs don't have a first scan bit. In these cases you may create your own first scan bit in ladder logic.
A first scan bit it a bit that the PLC turns on during the first scan of a PLC program when the PLC first powers up or when it changes from program mode to run mode. The bit is true for only the first scan of the program.
The first scan bit is useful for initializing retentive memory variables and resetting states when it is necessary that the equipment begin operation in a specific condition.
In some PLCs that support multiple programs and tasks there may be multiple first scan bits for each task which is true for the first time the task scans its programs. This distinction matters if you have a periodic or event triggered task that executes much later than the other tasks in the PLC.
Some PLCs don't have a first scan bit. In these cases you may create your own first scan bit in ladder logic.
Last edited:
iraiam
Member
We always use the first scan bit to ensure all logic is unlatched out of any AUTO RUN, MANUAL RUN, or other operating modes it may have been in when the power dropped out to the processor, or the processor was somehow reset.
We also have alarms that are associated with this bit, as a processor reset or having a brown out or anything of the sort causes an expensive downtime event. Maybe only 30 minutes, but at several hundred $$ a minute....
We also have alarms that are associated with this bit, as a processor reset or having a brown out or anything of the sort causes an expensive downtime event. Maybe only 30 minutes, but at several hundred $$ a minute....
MasterBlaster
Lifetime Supporting Member
You guys get an "A" on the OP's homework assignment.
iraiam
Member
Yay! I'm not even an engineer, just a lowly technician for 20+ years
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