Write a program for POS terminal to connect with payment gateway

I think what you need is a MOTO terminal (or virtual terminal) not a physical pos. With MOTO (Mail Order/ Telephone Order), you submit transactions manually by entering payment details into an Acquirer online Virtual Terminal. This feature is ideal for merchants who take mail or telephone orders from their customers. Because of the sensitive nature of these transactions whereby the merchants would have the customer's card info, you would have to be authorized to take in the customer's and card's info, depending on the policies of the Acquirer. Physical POS are for accepting in person payments where customers have their credit cards and card is swiped so that he doesn't have to divulge all the information to the merchant. There's no facility in a POS terminal for the seller to manually enter the card's number and other details because there are other sensitive info in the mag stripe needed for the transaction to be authorized at a physical POS. And besides the transactions you are referring is called Card Not Present transactions and is only possible for Mail Orders, Telephone Orders and internet purchases not possible at a physical POS. Both terminals (MOTO, and physical POS) need to registered with an Acquirer where you would need to have a merchant account with them. Check with your Payment Gateway provider whether they are providing a MOTO terminal for your particular purpose.