A Counter is a device which stores (and sometimes displays) the number of times a particular event or process has occurred, often in relationship to a clock signal. Counters are used in digital electronics for counting purpose, they can count specific event happening in the circuit. For example, in UP counter a counter increases count for every rising edge of clock. Not only counting, a counter can follow the certain sequence based on our design like any random sequence 0,1,3,2… .They can also  be designed with the help of flip flops. They are used as frequency dividers where the frequency of given pulse waveform is divided. Counters are sequential circuit that count the number of pulses can be either in binary code or BCD form. The main properties of a counter are timing , sequencing , and counting. Counter  works in two modes 
- Up counter 
- Down counter
Counters are broadly divided into two categories 
1. Asynchronous Counter 
In asynchronous counter we don’t use universal clock, only first flip flop is driven by main clock and the clock input of rest of the following flip flop is driven by output of previous flip flops. 
It is evident from timing diagram that Q0 is changing as soon as the rising edge of clock pulse is encountered, Q1 is changing when rising edge of Q0 is encountered(because Q0 is like clock pulse for second flip flop) and so on. In this way ripples are generated through Q0,Q1,Q2,Q3 hence it is also called RIPPLE counter and serial counter. A ripple counter is a cascaded arrangement of flip flops where the output of one flip flop drives the clock input of the following flip flop 
2. Synchronous Counter 
Unlike the asynchronous counter, synchronous counter has one global clock which drives each flip flop so output changes in parallel. The one advantage of synchronous counter over asynchronous counter is, it can operate on higher frequency than asynchronous counter as it does not have cumulative delay because of same clock is given to each flip flop. It is also called as parallel counter.

COUNTER GEMO DS204-230VAC:

48mm x 48mm  Enclosure front size
2x4 Digit Display, single PRESET, single OUT, up/down counter 
Counts switch, proximity switch & incremental encoder input 
Password protection, Selectable input frequency, Calibration constant; 0.001 ... 9.999 
Selectable decimal point; 1. .. 3. Digit, 7 input / 9 output function options, Adds “OFFSET” to Count Value 
OUT; latch or 0.1..999.9 seconds pulse 
Loads Count value & OUT status at the latest power failure after the first power on 
RESET via front panel 
EEPROM memory to store settings
Easy connection with plug-in connectors

Preset Up/Down Counter; Double Preset, Double Output, CP1, CP2, RESET inputs,
switch, proximity or encoder input (NPN/PNP/Totem Pole),
double display, 4 digits, calibration constant, SS outputs

COUNTER GEMO DS207-230VAC:

72mm x 72mm  Enclosure front size
2x4 Digit Display, single PRESET, single OUT, up/down counter 
Counts switch, proximity switch & incremental encoder input 
Password protection, Selectable input frequency, Calibration constant; 0.001 ... 9.999 
Selectable decimal point; 1. .. 3. Digit, 7 input / 9 output function options, Adds “OFFSET” to Count Value 
OUT; latch or 0.1..999.9 seconds pulse 
Loads Count value & OUT status at the latest power failure after the first power on 
RESET via front panel 
EEPROM memory to store settings
Easy connection with plug-in connectors

Preset Up/Down Counter; Double Preset, Double Output, CP1, CP2, RESET inputs,
switch, proximity or encoder input (NPN/PNP/Totem Pole),
double display, 4 digits, calibration constant, SS outputs

COUNTER GEMO DS217-230VAC:

96mm x 96mm  Enclosure front size
2x4 Digit Display, single PRESET, single OUT, up/down counter 
Counts switch, proximity switch & incremental encoder input 
Password protection, Selectable input frequency, Calibration constant; 0.001 ... 9.999 
Selectable decimal point; 1. .. 3. Digit, 7 input / 9 output function options, Adds “OFFSET” to Count Value 
OUT; latch or 0.1..999.9 seconds pulse 
Loads Count value & OUT status at the latest power failure after the first power on 
RESET via front panel 
EEPROM memory to store settings
Easy connection with plug-in connectors

Preset Up/Down Counter; Double Preset, Double Output, CP1, CP2, RESET inputs,
switch, proximity or encoder input (NPN/PNP/Totem Pole),
double display, 4 digits, calibration constant, SS outputs