Down Counter PLC Program

To study the working of Up Counter PLC program in Allen Bradley Programmable Logic Controller (PLC).

Down Counter

Down Counter (CTD) Instruction

In the above picture, there are totally three parameter,

COUNTER: C4:0 – Counter File name (TimerC5:0, C5:1,C5:2…)

PRESET – PRE : Limit value of COUNT-Up to how much it should count

ACCUMULATOR – ACC : Running Value of counter when condition turn ON.

Down Counter Data File

From the data file,along with preset and accumulator, we have few more bits,

CD: Countdown Bit-whenever the counter is enable makes this bit to go ON.

DN: Done Bit-When accumulator value equals preset value, done bit turns to ON.

UA: Counter Underflow – When accumulator value reached the limit value (-32767),it rolls back to +32767 for the upcoming counter operation, underflow bit turns ON, in this condition.

Notes:

UA – Update Accumulator Value-Only used when high speed counters are used in the program.

CU & OV – Used for Up Counter Function.

Down Counter Description Using PLC Program

I:0/0 is used to give input to counter and Preset value and accumulator value set to 5.

Counter Count Down Bit (C5:0/DN)

In the below Ladder logic,

Rung 0000

Having condition input I:0/0 which gives input to counter to perform counter down function.

Rung 0001

Having Counter CU Bit which enable only when counter is in function or when input to the counter turns ON and accumulator value counted down.

Rung 0002

Counter done bit turns on when accumulator value equals preset value.

Counter Count Down Bit

Counter Count Down Bit (C5:0/CD):

In the below Ladder logic,

Rung 0000

Having condition input I:0/0 which gives input to counter to perform counter function.

Rung 0001

Having Counter CU Bit which enable only when counter is in function or when input to the counter turns ON and accumulator value counted down.

Down Counter PLC Programming

In the below Ladder logic,

Rung 0000

Having condition input I:0/0 which gives input to counter to perform counter function. Accumulator value goes to negative if I:0/0 goes ON for 6th time in the below case,

Down Counter Ladder Logic

Counter Under Flow Bit (C5:0/UN):

In the Below Ladder Logic,

Counter accumulator value under flows when accumulator value reaches -32767 in Allen Bradley PLC Programming.

plc counter programming examples

In the below ladder Logic,

When we turn ON the I:0/0 ,accumulator value rolls back to 32768 and start counting down from 32767 to -32767.Counter underflow bit turns ON when this condition happen (Rung 003).

How to Use Counters in PLC Programming

Conclusion:

We can use this explanation to understand the working of Down Counter function in Allen Bradley Programmable Logic Controller (PLC).

Author : Hema Sundaresan

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1 thought on “Down Counter PLC Program”

  1. Hello Hema,
    Please, is there a way to hinder the countdown of the counter from reaching the negative count values without using a limits or comparison instructions?

    Reply

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