I've used Serial converters from Real-Time Automation. The435NBX series is made specifically for Allen-Bradley processors. You connect over ethernet and can have up to two serial devices per ethernet node. You configure the RTA to dump the data into a PLC address and monitor another PLC address for commands. It's pretty simple and straightforward, and the advantage is no complicated ASCII handshaking code in the PLC required. IF you're using A-B, that is.