CTD / CTDr

 Function Block A function block groups an algorithm and a set of private data. It has inputs and outputs. - Down counter.

Inputs

Input	Data Type	Range	Unit	Default	Description
CD	BOOL				Enable counting. Counter is decreased on each call when CD is TRUE.
LOAD	BOOL				Re-load command. Counter is set to PV when called with LOAD to TRUE.
PV	DINT				Programmed maximum value.

Outputs

Output	Data Type	Range	Unit	Description
CV	DINT			Current value of the counter.
Q	BOOL			TRUE when counter is empty (i.e., when CV = 0).

Remarks

• The counter is empty (CV = 0) when the application starts.
• The counter does not include a pulse detection for CD input.
• Use the f_trig or r_trig function blocks for counting pulses of CD input signal.
• CTDr, CTUr, and CTUDr function blocks operate exactly as other counters.
  • Exception: All Boolean inputs (CU, CD, RESET, LOAD) have an implicit rising edge A rising edge is the transition of a digital signal from low to high. It is also called positive edge detection included.

FBD Language Example

FFLD Language Example

• The CD is the input rung.
  • The output rung is the Q output.

IL Language Example

(* MyCounter is a declared instance of CTD function block. *)
Op1: CAL    MyCounter (CD, LOAD, PV)
FFLD "Free Form Ladder Diagram"     MyCounter.Q
ST     Q
FFLD     MyCounter.CV
ST     CV

ST Language Example

(* MyCounter is a declared instance of CTD function block. *)
MyCounter (CD, LOAD, PV);
Q := MyCounter.Q;
CV := MyCounter.CV;

See Also

• CTU / CTUr  
• CTUD / CTUDr