PLC Drum

Drums are a great tool when you’ve got a simple sequence of events that need to occur at set time intervals or as a result of predefined events.

Time Sequenced Drums turn bits on or off for set amounts of time.  Below is one such drum.  
Time that bits are held for is listed on the left side of the chart.  
Bits that are turned on or off are listed on the top of the chart (A, B, C, D, E & F).  
The sequence starts on from the top (the 9 second interval) and works it’s way down.

If this Drum were turned and left on it would operate for 54 seconds (9+15+5+7+6+12).

Lets look at what happens during those 54 seconds.  The * indicates that all values not mentioned are held at the value they had in the previous time interval:

 

Seconds    0 Р 9  A is turned on, everything else is turned off.

Seconds   9 Р24  B is turned on. *

Seconds 24 – 29 A & B are turned off, C & D are turned on. *

Seconds 29 – 36 C is turned off, E is turned on. *

Seconds 36 – 42 E is turned off, F is turned on. *

Seconds 42 – 54 D is turned off. *

 

Event Sequenced Drums differ from Time Sequenced Drums in 1 area.  Instead of holding states for a period of time, they hold states until an event occurs.  An event is when a specified bit transitions from low to high.

Some drums have a Continuous Mode option that causes the Drum to start over after it has reached the end of it’s sequence.