Cables vs Fibers

Start with the basics, then do a more specific search.

Optical fiber - Wikipedia, the free encyclopedia

See here:

http://www.modbus.org/

under Technical Ressources. The specs can be download for free and without registration.

Modbus is a communication protocol, you're not bound to a given physical layer. Can be electrical, optical or even wireless. Modbus is basic and its advantage is that if a communication driver does not exist it's less complex to implement than Profibus, CAN, etc.

Profibus is a fieldbus family widespread in continental Europe (originally pushed by Siemens).

Classical dedicated RTUs (typically for grid, water, wastewater, gas, etc.) are used less nowadays as they tend to be replaced by usual industrial remote I/Os (simply said, there could be a more detailed discussion, see also the DCS vs. PLC/SCADA discussions).

For the plastic fiber discussion, see the topic here for some basics:

http://cr4.globalspec.com/thread/98150/Use-of-Optical-Fiber-in-Motor-Control

(or are you the same person? I don't understand why anonymous posting is allowed, it doesn't make sense to me).

Plastic fiber is used for short distances and low bandwidths. One drawback is that there are lots of different proprietary connector systems. Glass fiber is more standardized but is also more fragile and much more expensive.

Also have a look at SERCOS, its first version from 1987 was already optical. Originally it was mainly used for fast communication in multi-axis servodrives (CNC, robotics, rotative printing,...).

Nowadays Ethernet-based communication tends to replace fieldbuses. The good side is that there's a lot of hardware available, drawbacks are fragile connector systems and lots of devices are not intended for industrial use and also when sharing networks which are not strictly dedicated to your industrial use it can become a mess because other departments can mess up your network.

(Not an exhaustive discussion of course, just some general points.)

-->fibermcgee&molly.com; they'll have everything you need to know.