O.P.: At LocalTalk speeds of 250k bits/sec, you may find that a software "bit banging" solution drops some incoming bits while the Arduino's processor is off doing other things, like talking to the Ethernet port.
Take a look at the Zilog Z8530 SCC http://en.wikipedia.org/wiki/Zilog_SCC and its modern equivalents. The IC was used in the original Macintosh and it took care of grunt work involved in dealing with LocalTalk, including checking (and generating?) CRCs. It is a bus oriented chip, so it may tie up quite a lot of I/O lines on the Atmel microcontroller in an Arduino.
Back in the day, many amateur radio enthusiasts used Macintoshes as packet radio terminals. The Z8530 IC made the Mac a much better candidate than an IBM Compatible PC of that era, because much of the work involved in packet could be offloaded to the SCC hardware. This avoided buying an expensive Z80 based packet "terminal node controller" (TNC). Most TNCs had Z8530s in them too!
Take a look at the Zilog Z8530 SCC http://en.wikipedia.org/wiki/Zilog_SCC and its modern equivalents. The IC was used in the original Macintosh and it took care of grunt work involved in dealing with LocalTalk, including checking (and generating?) CRCs. It is a bus oriented chip, so it may tie up quite a lot of I/O lines on the Atmel microcontroller in an Arduino.
Back in the day, many amateur radio enthusiasts used Macintoshes as packet radio terminals. The Z8530 IC made the Mac a much better candidate than an IBM Compatible PC of that era, because much of the work involved in packet could be offloaded to the SCC hardware. This avoided buying an expensive Z80 based packet "terminal node controller" (TNC). Most TNCs had Z8530s in them too!