Advantages and Difficulties
Until recently, a PLC (Programmable logic controller) would communicate with a slave machine using one of several possible open or proprietary protocols, such as Modbus, Sinec H1, Profibus, CANopen, DeviceNet or FOUNDATION Fieldbus. However, interest increased to use Ethernet as the link-layer protocol, with one of the above protocols as the application-layer (as in the OSI model).
Some of the advantages are:
- Increased speed, up from 9.6 kbit/s with RS-232 to 1 Gbit/s with Gigabit Ethernet over Cat5e/Cat6 cables or optical fiber
- Increased distance
- Ability to use standard access points, routers, switches, hubs, cables and optical fiber
- Ability to have more than two nodes on link, which was possible with RS-485 but not with RS-232
- Peer-to-peer architectures may replace master-slave ones
- Better interoperability
Difficulties of using Industrial Ethernet include:
- Migrating existing systems to a new protocol
- Real-time uses may suffer for protocols using TCP (but some use UDP and layer 2 protocols for this reason)
- Managing a whole TCP/IP stack is more complex than just receiving serial data
- The minimum Ethernet frame size is 64 bytes, while typical industrial communication data sizes can be closer to 1-8 bytes. This protocol overhead affects data transmission efficiency.
Read more about this topic: Industrial Ethernet
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