CTD

Down counter

The function block CTD (count down) allows you to program counting procedures.

Parameters

Input

CD (BOOL)

count down

the value 1 is subtracted from the current value CV for each rising edge detected at CD, except when LD is set or CV has reached the value zero.

LD (BOOL)

Load

with LD the counter state is reset to PV

PV (INT)

Preset value

is the value subjected to subtraction during the first counting procedure

Output

Q (BOOL)

signal output

is set if CV = zero

CV (INT)

Current value

contains the current subtraction result (CV = current value)
The value can be changed during counting operation by writing to the variable from the programming editor.

Time chart

Example

POU header

All input and output variables used for programming this function have been declared in the POU header. The same POU header is used for all programming languages.

	VAR
		initialize_count: BOOL:=TRUE;
			(*for setting the current value
to starting value*)
		count: BOOL:=FALSE;
			(*1 is subtracted from the current
value for each leading edge
detected at count*)
		load: BOOL:=FALSE;
			(*sets the curr_value back to 
set_value, if TRUE*)
		set_value: INT:=2;
			(*the starting value*)
		output_value: BOOL:=FALSE;
			(*is set, if the current value is
zero*)
		curr_value: INT:=0;
			(*the current_value*)
		ctd_1: CTD;
			(*instance of the counter*)
	END_VAR

POU body

If set is set (status = TRUE), the preset_value (PV) is loaded in the current_value (CV). The value 1 will be subtracted from the current_value each time a rising edge is detected at clock. This procedure will be repeated until the current_value is greater than/equal to zero. Then, signal_output will be set.

LD body

BODY
    WORKSPACE
        NETWORK_LIST_TYPE := NWTYPELD ;
        ACTIVE_NETWORK := 0 ;
    END_WORKSPACE
    NET_WORK
        NETWORK_TYPE := NWTYPELD ;
        NETWORK_LABEL :=  ;
        NETWORK_TITLE :=  ;
        NETWORK_HEIGHT := 7 ;
        NETWORK_BODY
B(B_FB,CTD!,copy_name,8,2,13,7,,?BCD?BLD?BPV?AQ?ACV);
B(B_VARIN,,output_value,6,5,8,7,);
B(B_VAROUT,,current_value,13,4,15,6,);
B(B_CONTACT,,set,3,4,5,6,);
B(B_CONTACT,,clock,3,2,5,4,);
B(B_COIL,,signal_output,20,3,22,5,);
L(7,4,8,4);
L(1,5,3,5);
L(5,5,8,5);
L(1,3,3,3);
L(5,3,7,3);
L(7,3,7,4);
L(13,4,20,4);
L(1,0,1,7);
        END_NETWORK_BODY
    END_NET_WORK
END_BODY

ST body

IF set THEN      (* first cycle *)
    load:=TRUE;        (* load has to be TRUE,
                        to set current_value to output_value *)
    clock:=FALSE;
END_IF;
copy_name(CD:= clock, LOAD:= set, PV:= output_value, Q=> signal_output, CV=> current_value);
load:=FALSE;    (* now current_value got the right value, load doesn't need to be *)
                (* TRUE any longer *);

IL body

If you want to call the function block in an Instruction List editor (IL), enter the following: