Tuesday, August 23, 2011

Communication Modbus and DeviceNet on ABB Scalable PLC





The RTU of Modbus is a protocol of open master or slave, and can be simply executed on serial interfaces. Many automation systems have interfaces of Modbus RTU as standard features or optional features, and are therefore simply able to correspond with the AC500.

Communication
The master sends a demand to the slave and next accepts the response by polling. Both interfaces COM1 and COM2 can activate at the same time as the interfaces features Modbus. The operating mode features Modbus is set with the tool of engineering.

Topology
The maximum of one slave and one master is likely with RS232, while a maximum of 31 slaves can be run with RS485. The maximum length of cable is 1.2 km with RS485 and 15 m with RS232.

Data transfer
Every telegram has a Max. 115.2 kB/s with 16-bit CRC added. The telegrams let process data to be read and written individually or groups.

DeviceNet
The protocol of CAN was initially built in for the automotive industry in European, so as to substitute high cost cabling by a reasonable network cable. It is also utilized in the automation field for process data transmitting between decentralized I/O modules, control systems, valves, drives, etc. The features of CAN a high level security of transmission, as great portions of the mechanisms monitoring have been realized directly in the chip of CAN. CANopen and DeviceNet use the structure of physical and the data transfer mechanisms of CAN. The difference lies in the protocols of transmission. The CANopen and DeviceNet can be utilized equally for the controller of AC31 and AC500 series and for devices of field-bus-neutral FBP.

Transmission Data
Two message types have been identified: transfer of I/O data and direct link. Transfer I/O data is utilized for process data time-critical, while the direct link can be used for messages of diagnostic.

Subscribers Bus access
The ID connection with the lesser address has upper priority on the bus. Data is sent out by the source, while the drops have similarly been specific during the phase of configuration.

CANopen
The bus works on the principle of master/slave with a master and up to 127 slaves. A protected cable of twisted pair is used, along with ISO 11898.

DeviceNet
The bus runs on the principle of multi-master and/or the master or slave, with up to 64 subscribers of bus. Two shielded type of cables twisted-pair are utilized: shaft cable for the drop cable and main line for the branch line.


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