For developing POS application below are the steps you should consider:
- Are you gonna card support schemes like VISA, MASTERCARD etc.
- Select appropriate device, if the answer is yes then you need a EMV Level1 & 2 certified device. There are many Chinese manufactures in market. Google them out. You can visit EMV Co Website to check if the kernal is certified. They should also provide you with demo application showing there device features.
- You will need a connecting server which will intern connect to a payment switch. The network communication will depend on on what the server provides, traditional POS devices(verifone,ingenico) connect on TCP/IP and use ISO8583 format(no http, no JSON, xml). But now, many switch servers provide a HTTP interface(Or there is at least an intermediary server which does that for you). If you're new to POS, I'll recommend the second way.
- After doing integration(read card data - send to switch - process payment - get response). If your answer to question 1 is yes, then you'll have to do ADVT (for VISA) and MTIP(for mastercard) Certification for your POS device. You need to capture the device and card logs, submit to VISA and Mastercard for review.
Note: For doing this, you need a sponsor bank(if you're gonna actually deploy your POS at sites). This applies in India, IDK about other countries.
You can use the JPOS library to get some knowledge.
In my experience, It takes at least an year for a POS system to be developed.