What factors should I consider when selecting an Ethernet PHY?
Ethernet is an interface specification specified in IEEE 802.3. The Ethernet physical (PHY) layer is one of the elements of IEEE 802.3. It is a transceiver component that is used to send and receive data or Ethernet frames. In the OSI model, Ethernet covers Layer 1 (Physical Layer) and part of Layer 2 (Data Link Layer).
The physical layer specifies the electrical signal type, signal rate, media and connector type, and network topology. Ethernet PHY can be mapped into this layer as shown in Table 1.
Table 1. OSI model
The PHY forms the physical interface and is responsible for encoding and decoding the data transmitted between a purely digital system and the signal transmission medium. It therefore represents the bridge between the digital and electronic connection layers of the interface.
The Data Link layer defines how communication takes place over the medium, and the frame structure for transmitting and receiving messages. This means that it defines how the bits coming from the wire are arranged to extract the data from the bit stream. In Ethernet, this is called the Media Access Control (MAC), and it is located next to the PHY, but in the Data Link layer. The MAC is usually integrated into a controller or switch.
The PHY can be a discrete component or integrated into the Ethernet controller. The simplified block diagram of Figure 1 shows the required Ethernet components and a discrete PHY.
Figure 1. Simplified block diagram of an Ethernet connection.
If a design must use a discrete PHY, there are several criteria that should be kept in mind when selecting the PHY.
Several important criteria to consider when selecting an industrial PHY
In industrial applications, data transmission and networking must be highly reliable and fail-safe over a wide temperature range. This applies to all components.
Network cycle time is the time required for a controller to collect and update data from connected devices. A PHY with low latency can reduce network cycle time, thereby improving network update time, which is critical for time-critical applications. This allows more devices to be connected to the network.
The working environment in industrial applications is often harsh. The PHY is connected to the cable directly or through small magnetic components. These connections may introduce interference (radiated or conducted), so the PHY must be able to withstand common external conditions.
EMC standards such as CISPR 32, IEC 61000-4-2 to IEC 61000-4-6 can be used as a benchmark to measure PHY specifications. A reliable PHY can help pass certification and avoid the often tedious redesign work.
Devices used in industrial applications are often protected against dust and moisture with IP65/IP66 ratings, which limits the airflow used to cool the electronics. At the same time, devices in industrial applications often need to withstand high temperature environments. In addition, in linear and ring topologies, two Ethernet connections are usually required, and therefore two PHYs are required, which doubles the PHY losses associated with data input and output. Therefore, a PHY with low loss should be selected to minimize the self-heating of the device.
ADI pays great attention to various industrial requirements when developing Industrial Ethernet PHYs, and has launched a number of reliable PHYs, including ADIN1200 (10 Mbps/100 Mbps), ADIN1300 (10 Mbps/ 100 Mbps/1 Gbps), and ADIN1100 (10BASE-T1L), to complement and improve its Industrial Ethernet ADI Chronous™ product line.
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