Information Technology Reference
In-Depth Information
Cable type
twisted-pair
Header/data size
1 to 13 or 52 bytes
Major benefits
large number of users
Nodes programmable
intelligent I/O modules
Primary applications
process control
19.2.2 WorldFIP
WorldFIP operates at 1 Mbps over twisted-pair cables, and is a reliable method of transmit-
ting variables (from sensors and to actuators) and messages (such as events, configuration
commands). It uses a bus arbitrator that broadcasts variable identifiers to all the nodes on the
network. This triggers the required node which produces the node to respond with the re-
quired value. All modules that need this value must then read it. Its main characteristics are:
•
It supports a distributed, decentralised database of variables.
•
It does not require node addresses as messages are broadcasted by a bus arbitrator, and
then the response is from the node which contains the processor parameter.
Its outline specification is
Speed
1 Mbps
Maximum nodes with/without repeaters
256/64
Maximum distance with/without repeaters
Greater than 10 km/2 km
Arbitration
bus arbiter
Cable type
twisted-pair
Header/Data size
1 to 128 bytes
Major benefits
distributed data base/ very deterministic
Primary applications
Real-time control/process/machine
19.2.3 CAN
Controller area network (CAN) was developed mainly for the automobile industry, and is
now popular in factory automation. It transmits at 1Mbps and uses twisted-pair cable for up
to 40 devices. Its main features are:
•
Nodes can communicate when there are no nodes communicating on the bus.
•
It uses a non-destructive bit-wise arbitration which allows fast detection of multiple ac-
cesses. This allows full use of the bandwidth. This differs from bus-topology LAN tech-
nologies, such as Ethernet, which detects collisions over long distances, and suffers from
propagation delays, and nodes may transmit many bits before they can determine if two
or more nodes are communicating at a time. In Ethernet, nodes back off from the net-
work when a collision occurs.
•
Message priority system, which is based on an 11-bit packet identifier.
•
Architecture can be many masters, and involves peer-to-peer communications or multi-
cast transmissions.
•
Automatic error detection, signalling and retries.
•
Short data packets of eight bytes.
Its outline specification is
