Belden Solutions Offer Optimal Performance for MODBUS Applications
Developed byModicon, MODBUS is a master/slave serial line communication protocol is used to interconnect intelligent automation devices. This protocol operates at layer 2 of the OSI model.
In a master-slave type system, the “master” node issues explicit commands to one of the “slave” nodes and processes responses. Typically, slave nodes will not transmit data without a request from the master node, and they will not communicate with other slaves.
At the physical level, MODBUS systems may use different physical interfaces (EIA-485, RS-232). The TIA/EIA-485 (EIA-485) Two-Wire interface is, however, the most common. As an option, the EIA-485 Four-Wire interface may be implemented. And when the system only requires short point-to-point communication, the TIA/EIA-232-E (RS-232) serial interface may be used as an interface.
MODBUS application layer messaging protocol, positioned at layer 7 of the OSI model, provides client/server communication between devices connected on buses or networks. The client role is provided by the Master of the serial bus and the slave nodes act as servers.
For applications that require high up-time, MODBUS Plus cable redundancy is offered on the dual cable PCI interface. A redundant cable capability allows MODBUS Plus to communicate over two independent cables, testing the integrity of both cables with every message. Should a problem occur with one of the cables, the system automatically switches over to the other cable. The faulty cable is then flagged as a statistic and it is visually flagged on the interface card. This redundancy allows a network or a plant to continue operation even with a cable out of service.
Using a RS-232 physical interface, the maximum distance would be 50 ft. at 10kbps. Using a EIA-485 physical interface, the maximum distance would be 4,000 ft. at 100kbps. A maximum of 247 nodes are possible.