How to integrate a self-written pos system with acquiring?

Question

Payment systems

Created a pos system for retail stores. Application for registration of sales is created on android.
Now you need to connect acquiring. The work should be as follows:
1. The seller dials the order
2. Indicates the type of payment "non-cash"
3. A signal is received at the terminal for payment by bank cards. The client enters the PIN code. Payment is made.
There are no problems with the technical side. We implement everything if we have the necessary documentation, but I can’t find such documentation anywhere yet. Tell me. How to be?