PLC communication:

The great majority of installed PLCs "service" a moderate amount of I/O (probably less than 128 I/O points). Furthermore, most of the I/O devices are wired onto PLC I/O modules that are installed in a "local" rack or chassis structure. In that arrangement, the I/O modules can communicate directly to the CPU module (which runs the PLC logic) via a wired backplane structure that connects all modules within the chassis.

Communication Between the CPU Module and I/O Devices

But, what if the input and output devices need to be at great distances (thousands of feet) from the CPU module? In such cases, major PLC manufacturers such as Allen-Bradley, General Electric and Groupe Schneider have created proprietary, high-speed networks to connect their PLC's CPU module to chassis units containing I/O modules, which may be thousands of feet away. These proprietary PLC networks are sometimes referred to as "remote I/O networks," which provide a reasonable description of their purpose. It is also possible to use new nonproprietary networks such as DeviceNet to allow a PLC to service I/O devices located at a distance. At the present time, the PLC acts as a "master" to the distantly-located "slave" devices in both of these categories of networks.

Figure 33. Master to Slave Communications

This simply means that the CPU always initiates and controls all communication to remote racks or other devices on the Network. The communication details of such networks are beyond the scope of this PLC overview.

Communication Between Multiple PLCs and Other Devices.

Major PLC manufacturers have also created proprietary networks to permit multiple PLCs of their own brand, plus certain other devices, such as PCs and operator stations, to share data. Examples of such networks include Allen-Bradley's Data Highway Plus and Groupe Schneider's Modbus Plus. Unlike the "remote I/O networks" mentioned earlier, there are not racks of I/O devices directly on these networks. Instead, these networks exist to connect the CPUs of multiple PLCs to each other and to PCs and other devices. These networks permit sharing and exchanging data collected by each individual PLC.

Because the CPU on each PLC may need to exchange data with any one of a dozen (or more) other PLCs on the network, each network must have a method of managing the communication traffic. Data must be sent between multiple PLCs or other devices without data "collisions" or confusion. Each network type has a unique protocol that establishes the "rules" of how communication will take place. If all devices on the network have the ability to initiate the transmission of data, the network is referred to has having "peer to peer" communication, rather than the "master/slave" arrangement that characterizes remote I/O networks.

Figure 34. Peer to Peer Communications

Many types of devices (PLCs, PCs, programming devices, Operator Interface (O/I)) can operate simultaneously on these types of networks. These networks are often used to report data from PLCs "up" to computers that are collecting plant-wide information. Many other types of "bridging" can exist between these networks and other networks or devices, but the discussion of those details is beyond the scope of this overview.

A Note about Electronic Operator Interface Products

PLCs can communicate with operator personnel via an electronic operator interface device (O/I). O/I products function just as their name implies - they allow the "operator" of a machine to "interface" with the PLC. This interface may include seeing the status of a counter, changing the set point on a timer, converting numerical data from Fahrenheit to Celsius, or any number of other operations.

Electronic O/Is can also replace standard control devices like pushbuttons, lamps and selector switches, thus decreasing the number of input and output devices that have to be wired to the PLC. Operator interface products are available to connect to the PLC via a wide variety of communication options, including connection to:

• a port on the PLCs CPU module
• a general-purpose proprietary network like Data Highway Plus
• a PLC remote I/O network
• a non-proprietary network like DeviceNet

The only wiring required for PLC-to-O/I communication is a single cable that links a port on the O/I to a port or node connection on the PLC or the network.  

PLC relay comparison

To see how far we have progressed since the time of the relay, consider the chart below. It summarizes the value of the PLC over the relay.

                                        Relay                                                           PLC