Basics of the buses
A group of Wiki-style web pages presented by Kunbus covers the basic history, characteristics and applications of the major fieldbuses and Ethernet-based protocols and technologies, including Profibus, Profinet, Ethernet/IP, Foundation fieldbus, Modbus TCP, DeviceNet, CC-Link, CANopen, EtherCAT, Ethernet Powerlink, AS-i, BACnet and others. They're located here
Kunbus Industrial Communications
This 40-minute video, "Industrial Network Fundamentals" by Eaton Videos, covers the basics, background and history of industrial communication networks, as well as their topologies and protocols. It's located here
Overview of Ethernet(s)
The 165-page, slide-based document, "Industrial Ethernet Technologies: Overview" by Martin Rostan, examines the technical principles, advantages and drawbacks, and recent history and trends affecting the primary Ethernet approaches. It was originally presented as part of an EtherCAT seminar series, and also has feedback from the covered organizations. The protocols include Profinet, EtherNet/IP. CC-Link IE, Sercos III, Ethernet Powerlink, Modbus/TCP and EtherCAT. It's available here.
Network introduction paper
A 22-page, academic-style paper, "Introduction to Industrial Control Networks" by Brendan Galloway and Gerhard Hancke, provides a thorough look at industrial networking, compares it to conventional/commercial networks, charts its history, compares stack organizations, and covers basic security issues. It's available here
Gerhard Hancke blog
Place for profibus, profinet
The source for all things Profibus and Profinet, PI North America demonstrates the benefits of using fieldbuses in general and Profibus and Profinet in particular, while assisting North American device manufacturers in developing and marketing products. Its website covers their technology, training, products, membership, news, forums and resources, including the Profiblog, application stories, white papers, videos and books.
PI North America
Foundation fieldbus, HART—and FDI
The three-year-old FieldComm Group combines the assets of the former Fieldbus Foundation and HART Communication Foundation. It's a global, standards-based, non-profit, member organization that directs development and implementation of communication technologies for the process industries. FieldComm Group is also responsible for development of Field Device Integration (FDI) technology.
Ethernet/IP VIA ODVA
EtherNet/IP lets users deploy standard Ethernet technology (IEEE 802.3 combined with the TCP/IP Suite) for industrial automation applications while enabling Internet and enterprise connectivity. It's administered by ODVA and its media-independent Common Industrial Protocol (CIP) and network adaptations, including EtherNet/IP, DeviceNet, CompoNet and ControlNet.
Two Fieldbus videos
These two, hour-long videos, "The Fieldbus Network, I and II" by Prof. S. Mukhopadhyay, Electrical Engineering Dept., Indian Institute of Technology Kharagpur, provide a thorough introduction to fieldbus network technologies and implementation methods. They're available here.
National Programme on Technology Enhanced Learning (NPTEL)
Classic comparison chart
The legendary Fieldbus Comparison Chart is located on several web pages, and may be a bit dated. However, it still provides an enlightening, side-by-side presentation of the various fieldbus protocols and technologies, especially their communication speeds, bandwidth requirements, cable lengths and other details. It's available in two locations: here and here.