Industrial Ethernet

Until recently, a PLC 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, there is now increasing interest in the use of Ethernet as the link-layer protocol, with one of the above protocols as the application-layer (see OSI model).

Some of the advantages are:

- Increased speed, up from 9.6 kbit/s with RS232 to 1 Gbit/s with IEEE 802 over Cat5e/Cat6 cables or optical fiber
* Increased overall performance
* Increased distance
* Ability to use standard access points, routers, switches, hubs, cables and optical fiber, which are immensely cheaper than the equivalent serial-port devices
* Ability to have more than two nodes on link, which was possible with RS485 but not with RS232
* Peer-to-peer architectures may replace master-slave ones
* Better interoperability

The difficulties of using industrial Ethernet are:

* Migrating existing systems to a new protocol (however many adapters are available)
* 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