With version 2, the Micro850 could only respond to ethernet messages from PLC's, HMIs, etc. But to have your CompactLogix read from it would be the same code you would use to have it read from another ComapctLogix.
However, version 4 of CCW and the Micro850 has added the ability for the Micro850 to execute it's own read and write messages on ethernet for Ethernet/IP or Modbus TCP - I haven't tried it yet myself but it should work.
Hope this helps,