I want my arduino to be a gateway between a RS485 serial network and an ethernet network
(meaning all information comming via serial should be send via Ethernet and vice versa).
My basic idea was to let the arduino listen to LAN-Events and handle inputs via RS485
(connected to the serial input trough an TI SN65176B) through the serialEvent() Event
as a kind of interrupt.
My questions are
Is it the right approach in general?
Could I use both kinds of Ethernet controller: ENC28J60 or W5100?
I would prefer the ENC28J60 because it's cheaper and there are smaller breakouts
available (for use with Pro mini), but I'm not sure if there are other reasons against
it (smaller cache, no Hardware TCP/IP-Stack, not stable enough for 24/7 etc).
The serialEvent() function does not act as an interrupt, and you don't need interrupts for this problem. Simply test each interface in turn for available data, reading and handling any data received. Depending what sort of conversion you need to do between the two interfaces you may be able to process each received byte separately, or may need to buffer incoming bytes until you have received a complete message.
You will need to do a little bit of connection administration on the Ethernet side to establish and maintain the connection to whatever remote server(s) you're communicating with, but that would be straight forward.