The input to a counter is a pulse train. The pulses may have to be conditioned to fit the input circuit of the counter. Some counters (those on the Microlink 551 and 302x for example) accept TTL voltages. These will normally give a 0 (logic low) output for inputs less than 0.8 V and a 1 (logic high) output for inputs greater than 2.4 V.
The Microlink 152x range of counters are designed for 24 V logic but will work on 5 V logic signals. Because they are isolated additional variability is introduced. Nominally they will switch from logic low to logic high between 3 and 4 V. They are all tested so that the outputs are logic high for 5 V inputs, and all outputs are logic low for inputs less than 2 V. These levels can be changed by adding or modifying resistors.