Page 338 - Programmable Logic Controllers, Fifth Edition - Mobile version

Processor
Processor
Processor Com Network interface module Processor Com Connectors Drop line
Com
Com
Trunk line

Main trunk line Media attachment Termination Tee tap

Figure 14-29 Bus topology network.

network. The operation of a bus topology network can be analog devices are called byte-wide bus networks. These
summarized as follows: networks can transfer 50 or more bytes of data at a time.
• Uses a single bus trunk cable to which individual Device bus networks that interface only with discrete
PLC nodes are attached by a cable drop that taps off devices are called bit-wide bus networks. Bit-wide net-
the main cable. works transfer less than 8 bits of information to and from
• Each PLC is interfaced to the bus using a network simple discrete devices.
interface module that is attached using a drop cable Process bus networks are capable of communicating
or connector. several hundred bytes of data per transmission. The ma-
• Due to the nature of the bus technology, and the jority of devices used in process bus networks are analog,
whereas most devices used in device bus networks are
way the data are transmitted on the network, each discrete. Process bus networks connect with high-level
end of the bus must be terminated with a terminat- information devices such as smart process valves and
ing resistor. flowmeters, which are typically used in process control
• As the data move along the total bus, each PLC applications. Process buses are slower because of their
node is listening for its own node identification large data packet size. Most analog control devices are
address and accepts only information sent to that used in controlling such process variables as flow and
address. temperature, which are typically slow to respond.
• Because of the simple linear layout, bus networks A protocol is a set of rules that two or more devices
require less cable than all other topologies. must follow if they are to communicate with each other.
• No single station controls the network and stations Protocols are to computers what language is to humans.
can communicate freely to one another. This book is in English, and to understand it, you must
• Bus networks are very useful in distributive control be able to read English. Similarly, for two devices on a
systems, because each station or node has equal network to successfully communicate, they must both
independent control capability and can exchange understand the same protocols.
information at any given time. A network protocol defines how data is arranged and
• Another advantage of the bus network is that you coded for transmission on a network. In the past, communi-
can add or remove stations from the network with a cations networks were often proprietary systems designed
minimum amount of system reconfiguration. to a specific vendor’s standards; users were forced to buy
• This network’s main disadvantage is that all the all their control components from a single supplier. This is
nodes rely on a common bus trunk line, and a break because of the different communications protocols, com-
in that common line can affect many nodes. mand sequences, error-checking schemes, and communi-
cations media used by each manufacturer. Today, the trend
I/O bus networks can be divided into two categories: is toward open network systems based on international
device bus networks and process bus networks. Device standards developed through industry associations.
bus networks interface with low-level information devices Most PLCs adhere to the protocols established by the
such as pushbuttons and limit switches that primarily International Organization for Standardization, or ISO.
transmit data relating to the on/off state of the device and The Open Systems Interconnection (OSI model) is a set
its operational status. Device bus networks can be further of seven layers that define the different stages that data
classified as bit-wide or byte-wide buses. Device bus must go through to travel from one device to another
networks that include discrete devices as well as small over a network. The OSI model takes into account the

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