Pick a bus, any bus

For most users who are designing their own machine, bus choice really comes down to three choices: RS-485 (yes, the old standby), CANbus, and Ethernet. Firewire and USB have enough problems associated with them, and enough of a learning curve, that there is very little activity at the user level in constructing systems using these buses. Again, if you are going to purchase a proprietary motion system that uses a network bus, then some of these considerations don’t really matter. But if you are going to construct a machine control system from an open, expandable, bus, those are the choices that you will most likely select from.

CANbus is probably the most widely used bus for “inside-themachine” networking. CANbus is faster than RS-485, but slower than Ethernet. What matters is whether it is adequate for the application at hand. And for a large number of users, it is.

Ethernet is the bus of the present, and the future. What it lacks in determinacy, it makes up for in speed. Its biggest drawback is the complexity of protocol layers. This may be fine when communicating with multi-axis robots or intelligent controllers, but becomes a nuisance when the distributed module is supposed to be simple and cheap. Still, Ethernet is pushing further and further into the box. New microprocessors are being released which can perform real time I/O tasks as well as host a complete Ethernet protocol stack.