Is there an open source client software for payment terminals?

Question

Payment systems

We needed client software for open source payment terminals.

Answer 1

Unlikely. Usually everything is made to order.

Answer 2

Usually, a payment terminal is rented from a bank and the client simply does not have access to the software part.
The terminal manufacturer usually provides an SDK with sample programs.
In general, if you search https://github.com/search?utf8=%E2%9C%93&q=POS+terminal , you can find a lot of things