Yes, I once implemented the whole thing using a client script. When a POS payment is selected using card payment, the amount is sent to the card terminals. I am currently trying to implement the whole thing as a standalone integration for ERPNext, but I am still in the development phase.
With the script, the process is such that when a card payment is made, the amount is sent to the card payment device, but no confirmation is received as to whether the payment has actually taken place. This should no longer be the case in the extension.
But so far, the whole thing has been running without any major problems for almost 1.5 years