Data Communications
Data Communications pocket guide - you have to register but it is worth the effort! Thanks to IDC Technologies
Industrial Data Communication - Westermo Handbook -Theoretical and General Applications, This158 page handbook is a little commercial but has good information, it is a large download though so be prepared to wait if you are not on broadband.
RS-485 Serial Data Communication - Physical Layer for Factory Automation and Process Control Networks - Mark E. Hazen and PowerPulse.net
Protocols Directory - protocols.com
RS-485 Serial Data Communication – Physical Layer for Factory Automation and Process Control Networks- By Mark E. Hazen and Intersil Corporation
Understanding Some Basic Recommended Standards for Serial Data Communications - A comparison of RS-232, RS-422 and RS-485- By Mark E. Hazen and Intersil Corporation
Technical Resource and Course Web Site for Data and Computer Communications- This site has a significant amount of useful information - williamstallings.com