The ControlNet network uses the Common Industrial Protocol (CIP) to combine the functionality of an I/O network and a peer-to-peer network. ControlNet take precedence over program uploads and downloads and messaging. Supports a maximum of 99 nodes.
DeviceNet is mainly used in industrial and process automation. It is based on CAN technology.
It is a low-cost communication link to connect industrial devices to a network and eliminate expensive hard wiring. Power and communication supplied over a 4-wire bus. Supports up to 62 devices on the same bus network.
ModBus is an open, serial communication protocol based on the master/slave architecture. The bus consists of a master station, controlling the communication, and of a number of slave stations.
MODBUS is an application layer messaging protocol, positioned at level 7 of the OSI model, that provides client/server communication between devices connected on different types of buses or networks. MODBUS is used to monitor and program devices; to communicate intelligent devices with sensors and instruments; to monitor field devices using PCs and HMIs. MODBUS is an ideal protocol for RTU applications where wireless communication is required.
Modbus offers two basic communication mechanisms:
* Question/answer (polling): The master sends an inquiry to any of the stations, and waits for the answer.
* Broadcast: The master sends a command to all the stations on the network, and these execute the command without providing feedback.
Serial Transmission Modes of MODBUS Networks
The transmission mode defines the bit contents of the message bytes transmitted along the network, and how the message information is to be packed into the message stream and decoded. The mode of transmission is usually selected with other serial port communication parameters as part of the device configuration.
Standard MODBUS networks employ:
1. ASCII Mode: Each character byte in a message is sent as 2 ASCII characters. This mode allows time interval of up to a second between characters during transmission without generating errors.
2. RTU Mode: Each 8-bit message byte contains two 4-bit hexadecimal characters, and the message is transmitted in a continuous stream. The greater effective character density increases throughput over ASCII mode at the same baud rate.
PROFIBUS-DP purpose is for larger devices like PCs and PLCs to talk with multiple smaller devices like sensors, drives, valves, etc. It uses RS-485 for transmission of data. It uses a shielded twisted pair cable and enables data transmission speeds up to 12 Mbit/sec.
A maximum of 9 segments (trunk line) are allowed on a network. The devices are the branches coming off the trunk line. Up to 32 individual devices can be connected to a single segment. That number can be expanded up to 126 if repeaters are used. Each PROFIBUS segment can be a maximum of 1200 meters in length. There are 10 defined communication speeds and each has a maximum defined cable length that’s permitted.
PROFIBUS uses a master/slave configuration for communication. It is usually a single master device (aPLC) that talks with multiple slave devices (sensors). The master devices poll the slaves when
they have the token. Slave devices only answer when asked a question. They are passive and the master can be said to be active. The slave devices just collect data and pass it to the master device when asked to do so.
Ethernet is one of the most widely implemented LAN architecture. It uses a bus, star or tree topologies. It uses the CSMA/CD access method to handle simultaneous demands. It supports data transfer rates of 10 Mbps, Fast Ethernet (100 Base-T)- 100 Mbps, and Gigabit Ethernet – 1000 Mbps.
Carrier Sense Multiple Access/Collision Detection (CSMA/CD)
This is a system where each computer listens to the cable before sending anything through the network. If the network is clear, the computer will transmit. If some other node is already
transmitting on the cable, the computer will wait and try again when the line is clear.
Most manufacturers who offer Ethernet compatibility to implement supervisory functions over equipment controlling plant floor functions use a transmission control protocol/internet protocol (TCP/IP) for layers 3 and 4 of the OSI model. Some PLC manufacturers offer programmable
controllers with TCP/IP over-Ethernet protocol built into the PLC processor. This allows the PLC to connect directly to a supervisory Ethernet network. Note that the PLC can also have a control network with other PLCs.