Your motion network

So with those introductions under our belt, lets get down to brass tacks and start naming names. Here are brief summaries of the most popular buses used for motion networking today. These networks are Ethernet, CANbus, RS485, and FireWire. Rest assured that there are many other choices available, in particular Profibus (popular in Europe), Foundation field bus, USB (the reigning PC interconnect bus-of-choice), SERCOS, Lon- Works, and others. But for various reasons, these buses are not in the mainstream of today’s general-purpose designs, with the possible exception of Profibus in Europe.

Ethernet is not deterministic in its native, full protocol mode, but can be made more deterministic by stripping some of the higher levels away. Ethernet has speed ranges from 10MB/sec to 1 gigabit per second and is sold in very high volumes, so costs are low for chips and hardware. For motion applications, Ethernet is commonly used in Semiconductor Capital Equipment industry, and other high value, high performance products.

CANbus started as an interconnect system for the automobile world. It evolved to be a popular device-level interconnect bus for general industrial use, and many motion users have adopted it due to its robustness and ease of use. DeviceNet and CanOpen are protocols on top of CANbus that are sometimes used to gain access to standard, off-the-shelf sensors and actuators. CANbus is used in a wide range of industries including medical automation, packaging, liquid dispensing, general automation, and others.

RS-485 and its related cousins RS-422, and RS-232 are surprisingly popular for use in communicating between motion modules. Many of the existing all-in-one “integrated motors” now on the market use RS-485. What they lack in protocol sophistication and automatic error checking, they make up for in simplicity and low cost. RS-485 and similar serial buses are used in industries whenever performance requirements are modest, and cost sensitivity is high.

FireWire (also called IEEE 1394) was developed by Apple Computer for use in video processing. It has attracted motion-world attention because of its high speed and determinism. FireWire has been adopted by a number of motion vendors as the “ideal” motion bus but users who are building their own network-based control system do not commonly use it. FireWire has a length limitation of 10 ft (depending on data rate, and which can be extended with repeaters) and if its use in the PC world declines, it is unclear how much ongoing support there will be from major chip vendors. FireWire is commonly used in highly synchronized applications such as machine tools.