Fairbanks FB2550 SERIES - Manual (Page 113)

Section 5: Input/ Output
07/2012 113 51254 Rev. 3
5.14. FIELDBUS PROTOCOLS AND FORMATS
5.14.1. TRANSMISSION METHODS
Communication protocols are simular to conversations; there are several different
languages and methods used.
– PROFIBUS, MODBUS, INTERBUS-S and ETHERNET use a method
called "source-destination" communications. The message packets have
destination information in them, and the Fieldbus passes a token from node to
node in a timed fashion.
– DEVICENET, CONTROLNET , and CAN use a broadcast, producer-
consumer model for communications. Messages are broadcast to all
nodes, and each node only "hears" messages intended for it.
5.14.2. COMMUNICATION FORMAT
Another major difference among fieldbuses is the format of the communications
themselves.
– DEVICENET and CAN open messages are eight bytes long.
– PROFIBUS is "word-oriented", and can have an up to 256-byte "stack" per
message.
COSTS vs. SPEED
– PROFIBUS and CONTROLNET are very fast networks – 12 megabits
per second and 500 Mb/s, respectively. They are much more expensive to
operate.
– DeviceNet is less expensive.
5.14.3. HANDLING NETWORK TRAFFIC
FIELDBUSES also handle network traffic in different ways.
– DEVICENET and CAN use "non-destructive bitwise arbitration." When
two messages collide, the higher priority message goes first. If the two are
equal priority, there is a mechanism within DeviceNet (as well as CAN) that
decides which one should go first.
– When a collision occurs in ETHERNET , all devices "back off" and re-send
their messages, which results in slower transmissions.