Communication Networks, Buses Tie Controls to Real World
As controls and automation become more distributed and integrated, industrial communication networks and buses are becoming more crucial because they link controls with real world, in-the-trench, manufacturing processes. However, due to growing choices of networks, protocols, buses, and node connections, it is helpful to understand the basic aspects of each network or bus before picking an overall architecture.
Physical topologies include linear buses (with drops), and daisy-chained, ring (where all nodes are connected in a physical ring), star, and mixed types. Each has its own advantages and drawbacks.
Bus arbitration schemes include master-slave and peer-to-peer (CSMA or collision sense/multi-access, and token-passing). On a master-slave network, a master node manages network access, typically polling each node and granting access. Peer-to-peer networks share bus ownership with all nodes having equal access. Token-passing buses (token ring/token bus) give token holders bus ownership until the token is passed.
CSMA allows any network node to exchange data if the bus is idle. Each node monitors transmission, and temporarily backs off if it finds a collision. This makes bus use of bandwidth efficient, but can sometimes degrade throughput in heavily loaded networks.
Many buses are able to broadcast status data periodically, and then any node can retreive what it needs and use it immediately. This is known as producer/consumer technology. ControlNet, DeviceNet, Fieldbus Foundation, and WorldFIP protocols are based on producer/consumer industry technology, which permits all nodes on the network to have this simultaneous access to the same data from a single source.
Industrial communication networks can be grouped into three basic categories: general-purpose networks, fieldbuses, and device buses (also referred to as bit-level buses). Various physical topologies and arbitration schemes exist for each type.
General-purpose communication networks provide broadcast and point-to-point messaging between nodes, and perhaps to other networks via bridges. Networks such as Ethernet, ARCNet, FDDI, IBM Token Ring, and MAP, are used for data gathering, interprocess exchange of control data and sequencing information, and remote access.
Fieldbuses are optimized to exchange periodic data--also known as producer/consumer information--with I/O devices, while providing time-slices for point-to-point messaging and network management tasks. Both messaging and network management capabilities assist in remote coordination of I/O devices (such as configuration and diagnostics) among other possibilities. Many provide bus electrical isolation among nodes, and add device profiles which virtualize equipment from different manufacturers, thus providing a common access model in applications. Fieldbus Foundation, Profibus, and SP50 are typical examples.
Device buses, such as AS-I (AS Interface) and Seriplex, provide cost-effective connectivity to I/O devices. Although the cabling and data rates are not typically advanced, actual throughput can be quite high due to the reduced frame size and simplified bus arbitration and addressing schemes.
Other networks and buses tend to fall between these categories. Newer, so-called sensor buses, such as DeviceNet, CAN, and SDS offer some benefits of a fieldbus, while keeping connectivity costs closer to device buses. Some offer remote configuration of I/O devices and programmability.
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