Linux Network device driver for Rs232

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http://stackoverflow.com – I want to write Linux device driver for network based Rs232 hardware. This hardware is connected to RTL8139 network interface card through Ethernet cable.The PCI RTL8239 card interfaced with Intel X86 machine . With this situation, How can i write driver for Rs232 network hardware? How to transfer data between kernel to Rs232 network hardware? How can i implement? (HowTos)