The technological network is physically a standard Ethernet network. It is used for communication of the process stations between each other, with the management station, and for data exchange with 3rd party systems. General rules for Ethernet network design apply. It is to be noted that the network may be exposed to stronger EMC load (e.g. because of variable speed drives, or PV inverters) than office networks, and therefore optical links are preferred at longer distances.
The network has a star structure, where at the ends of the branches there are process stations and other network devices, and in the center there are active elements – switches. The process stations are connected to a switch by standard UTP (unshielded twisted pair) or FTP (foil-shielded twisted pair) cables, category 5E or 6.
In the panel, the cable is terminated either by a standard Ethernet socket with a RJ45 connector for base plate installation, or in a patch panel which contains more sockets in a single frame. The process stations are connected to the sockets using patch cables, ready-made RJ45 straight cables of different lengths, usually supplied and installed by the panel supplier.
If there are more process stations in a single panel, it has to be considered if more Ethernet sockets have to be installed with separate cables from the main switch, or if a single cable and a separate switch directly in the panel connecting all process stations is a better solution.
The maximum length of a single Ethernet cable is 100m, which may be a limiting factor. Then it is possible to insert an active element (switch), or use a optic fibre link. The optic fibre may be more expensive than a UTP cable, but it is less susceptible to EMC immissions and provides higher possible cable length (up to 20 km or more). To convert the signal from a metallic link to optic fibre and vice versa, media converters are used, which are available also in industry grade versions (installation on a DIN rail, power by low SELV voltage).
Check the compatibility of all passive and active network devices, or consult an IT expert when designing the network.
At some projects, it is possible to take use of the local IT infrastructure or its parts. It is also possible (and advisable) to outsource the entire network to an IT company which supplies hardware and installation of all network components, and hands over a completely functional network system inclusive all tests and protocols with measured parameters of all installed sockets. The interface between the control system designer and the IT infrastructure designer is basically a floor plan with a list of panels and deployment of the network sockets.
To keep the network secure, it is recommended to install it as a separate system which is connected to the building network or/and to the Internet using a router with firewall, configured according to the recommendations of the local IT administrator.
There may be different components of the control system connected to the network:
- process stations
- management stations
- wireless access points
- routers to connect to other networks
- terminal servers for remote connection of I/O buses
The devices must be addressed properly to be able to communicate to each other.
IP version 4 addressing is used, which provides addresses as 4 numbers ranged 0…255, separated by dots. For private networks, which is the case of technological networks, the following ranges are reserved:
10.0.0.0 – 10.255.255.255
172.16.0.0 – 172. 16. 255.255
192.168.0.0 – 192.168.255.255
The exception may be embedding of the process stations into a customer network with its own numbering rules, e.g. as part of an international infrastructure. Consult the IT administrator for details.
If the control system designer is responsible for the network numbering plan, addresses of 192.168.1.10 and higher are usually used, with network mask of 255.255.255.0. It is also possible to state in the project that the network numbering is up to the software engineer who commissions the site and who should consult the numbering plan with the customer or end user.