Two established technologies come together in the digitalization of process automation: Ethernet and shielded, twisted pair cables. They enable a flat, fast and fully networked infrastructure that is attracting interest from suppliers and users alike.

However, Ethernet in process plants initially raises numerous questions: What happens to the existing cabling? What cable lengths are achievable? What tools are available for accessing the instrumentation? What about intrinsic safety in hazardous areas?

Performance data and infrastructure

Four aspects must be precisely defined in order to create a completely open infrastructure that offers the interoperability that users expect from Ethernet.

Firstly, the standard for single-pair Ethernet IEEE 802.3 cg defines data communication as 10BASE-T1L. The bandwidth of 10 Mbit/s allows, for example, the updating of radar curves every two seconds and supports users during time-critical phases, such as those that can occur during the commissioning of tank sensors. This bandwidth is also crucial for automatically downloading, saving and restoring device configurations. This significantly reduces the risk of human error.

The Ethernet APL port profile specification also defines the power supply. The planner simply selects switches and devices with matching power classes. For example, Ethernet-APL defines standard power levels for intrinsically safe device connections such as Ex ic IIC for Zone 2/Div. 2 or Ex ia IIC for Zone 0 or 1/Div. 1.

Thirdly, the Ethernet APL port profile specification defines connections, cables and accessories such as overvoltage protection. Changes to the cable infrastructure are quite complex and costly. It is therefore crucial to make the right decisions right from the start. The table on page 48 shows standardized parameters for Ethernet APL that are compatible with fieldbus communication technologies. It is advantageous if planners and system operators specify parameters from the outset that are tailored to the requirements of the respective process system. Reliable communication requires shielded two-core type A cables. If the existing cables are to be used to set up an Ethernet network, it makes sense to ensure that they are compatible with the specifications (see table) in order to avoid risks. This check is simple, as Ethernet-APL only requires two wires and not four or even eight. It is possible with standard tools and can be carried out by installers and office colleagues.

One provider has even tested 10 Mbit communication via an unshielded twisted pair cable. However, this was done under laboratory conditions - in practice, this is strongly discouraged. With unshielded cables, communication is not protected against crosstalk and electromagnetic interference. The required stability of communication is therefore not guaranteed in the long term.

The fourth aspect is intrinsic safety, where a new chapter of the well-known standard for explosion protection defines the same simple planning and validation approach that users of FISCO are familiar with (IEC TS60079-47, 2-WISE).

Direct line to each field device

Ethernet-APL Field Switches allow point-to-point connections to field devices according to standardized track definitions. The maximum cable length is 200 m with optional intrinsic safety. The 'FieldConnex' series from Pepperl+Fuchs also includes Ethernet APL switches for DIN rail mounting. Robust housings allow installation in the immediate vicinity of a field device in Zone 2 or a remote I/O station. Power supply and communication are segmented via point-to-point connections, allowing for a simple topology. Even with repeated connection and disconnection - for example during maintenance work on or when replacing instruments - this infrastructure is highly resistant to interference.

Replacing a field device is simple: as soon as the new device is connected, it is automatically switched on and recognized and identified by the asset management system. This allows asset management systems to automatically restore the configuration. Standard Ethernet cables can be used between the control center and the distribution box. With the help of fiber optic cables, distances of up to 2,000 m can even be bridged. Standardized Ethernet concepts apply for redundancy, such as the Media Redundancy Protocol (MRP), which is also used for Profinet.

A common network structure

Process plants are often in operation for several decades. During this time, the technology of the field instrumentation used often changes. Ethernet provides the basis for connecting different generations of technology under a common network structure. Appropriate protocols allow engineering independent of the data source.