KENWESONGA

Hi the counting signal will come direct on my digital input

Sorry, but I don't understand this. In the topic title you wrote "Modbus Master".

What has the digital input to do with "Modbus Master"? 

You have edit your answer as I wrote my answer.

The counting of fast input signals are only possible up to 5 kHz if the  fast input is connected to the counting input of a up/down counter. There is the maximum 999,999.

You can cascade two up/down counters using a third up/down counter. But the transfer to the second up/down counter takes place in a cycle. This can cause counts to be lost.