# IEC up/down counter

Canonical URL: https://eloragrid.ai/symbols/iec-up-down-counter
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: CTR +/-
Drawing use: Schematic
Reference families: IEC, Manufacturer

## Symbol convention

Convention: IEC
Form: CTRm element with M dependency selecting count up (+) and count down (-)
Qualifying-symbol and dependency conventions as explained in TI's public overview of IEEE Std 91-1984, which incorporates IEC 617-12 work. Independent educational drawing; not certified IEC geometry or an IEC database identifier.

IEC counter whose mode inputs select counting up with + or down with -, drawn with M dependency.

## In plain English

### What does IEC up/down counter mean?

An up/down counter is shown with a CTR additional drawing mark and a clock input labelled with both counting directions, for example 2+ and 1- under mode control. A direction input produces M1 and M2 modes, so the same clock counts up in one mode and down in the other. Load and enable functions use further dependency labels.

### A simple example

Typical applications include position tracking from pulse inputs, reversible totalising and digital set-point adjustment. The drawing should identify the direction source and whether direction may change while clock pulses are present.

### What the drawings help you do

The symbol does not indicate how overflow or underflow is signalled beyond any CT= outputs shown. Check ripple clock or max/min outputs on the device.

### This particular reference

IEC counter whose mode inputs select counting up with + or down with -, drawn with M dependency. A single-direction CTR counter shows only + on its clock.

### Terms

- Signal: Electrical information passed between devices, rather than necessarily the main power supply.
- Schematic: A drawing showing function and connections rather than physical appearance.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [Overview of IEEE Standard 91-1984: explanation of logic symbols (SDYZ001A)](https://www.ti.com/lit/ml/sdyz001a/sdyz001a.pdf)
- [SN74HC191 synchronous 4-bit up/down binary counter](https://www.ti.com/lit/ds/symlink/sn74hc191.pdf)


## What it represents

An up/down counter is shown with a CTR qualifier and a clock input labelled with both counting directions, for example 2+ and 1- under mode control. A direction input produces M1 and M2 modes, so the same clock counts up in one mode and down in the other. Load and enable functions use further dependency labels.
Typical applications include position tracking from pulse inputs, reversible totalising and digital set-point adjustment. The drawing should identify the direction source and whether direction may change while clock pulses are present.
The symbol does not indicate how overflow or underflow is signalled beyond any CT= outputs shown. Check ripple clock or max/min outputs on the device.

## How to read it

- Find the mode input that selects direction.
- Check CT outputs for terminal counts in each direction.

## Information to show or reference

- Show both + and - functions with their mode numbers.
- Label the direction input clearly.
- Show cascade outputs for multi-stage counters.

## Important distinction

A single-direction CTR counter shows only + on its clock.

## Facts

- Direction notation: + (up) and - (down) under M dependency
- Example device: Synchronous 4-bit up/down binary counter
- Artwork scope: Original educational illustration; consult the applicable standard and the selected device documentation for the exact graphic

## Public references

- [Overview of IEEE Standard 91-1984: explanation of logic symbols (SDYZ001A)](https://www.ti.com/lit/ml/sdyz001a/sdyz001a.pdf) — Texas Instruments. Counting up and down inputs and M dependency.
- [SN74HC191 synchronous 4-bit up/down binary counter](https://www.ti.com/lit/ds/symlink/sn74hc191.pdf) — Texas Instruments. Manufacturer up/down counter documentation.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.