SLAAEM2 October   2024 AM2434

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 Real-Time Communication in Factories
    2. 1.2 Industrial Protocols
    3. 1.3 Serial and Ethernet-Based Communication Protocols
  5. 2Industrial Protocols
    1. 2.1 Ethernet-Based Communication Protocols
    2. 2.2 Network Topologies
    3. 2.3 OSI Layer Model
    4. 2.4 Industrial Ethernet System Block diagram
      1. 2.4.1 Two-Port Device
      2. 2.4.2 One-Port Controller
    5. 2.5 Ethernet Physical Layer (PHY)
    6. 2.6 Media Access Controller (MAC)
      1. 2.6.1 Device MAC
      2. 2.6.2 Controller MAC
    7. 2.7 Industrial Protocol Stacks
    8. 2.8 Industrial Communication Software Development Kit (SDK)
    9. 2.9 EtherCAT Device Example Using the AM243x Processor
  6. 3Conclusion

2.5 Ethernet Physical Layer (PHY)

Industrial Ethernet protocol can use 10BASE-T, 100BASE-TX and 1000BASE-T, with the majority of protocols currently using 100BASE-TX. The Ethernet cabling is using 2-twisted pairs for 10BASE-T and 100BASE-TX and 4-twisted pairs for 1000-BASE-T. The Ethernet data send on the cable pairs is full duplex, which means that the PHY can receive and transmit at the same time. Those Ethernet standards require a cable reach of 100m between two field devices. If a longer cable reach is needed, then a Ethernet hub or Ethernet switch has to be inserted into the Ethernet line.

For 10/100Mbps, shown in Figure 2-6, there are two pairs of Ethernet cable used: one pair is used for transmit and the second pair is used for receive.

PHY 100mbit Figure 2-6 PHY 100mbit

For 1000Mbps, there are four pairs of Ethernet cable used. In Figure 2-7, the PHY transmit and receive simultaneously on all four pairs.

PHY 1gbit Figure 2-7 PHY 1gbit

The data transfer bandwidth depend on the established LINK speed, and is 10Mbps, 100Mbps or 1000Mbps. The Ethernet PHYs for those IEEE standards do support multi-speed. This means that a 10/100Mbit PHY support the two speeds of 10Mbps and 100Mbps, while a 10/100/1000Mbit PHY supports in addition also 1000Mbps.

Table 2-2 lists industrial Ethernet protocols required for specific PHY features.

Table 2-2 Industrial Ethernet Protocol PHY Features
Feature Description Protocol example
Fast-Link-Down (FLD) The PHY needs to be able to drop an active link when receive errors occur. Such feature is mainly important to industrial protocols that need to loop back Ethernet frames to the controller, like in EtherCAT. A device with a link-down needs to switch to loopback mode once the link-down is detected. FLD works with PHY LINK LED state, which is fed back to the MAC. Note that alternatively the LINK state is read out by the MAC using MDIO/MDC protocol, and this may take several microseconds to detect. EtherCAT requires <15μsec of FLD
Low transmit and receive latency PHY latency has to be as low as possible, because in line topology the latency between PLC and last device does sum up. EtherCAT requires a port to port pass through latency of <1μsec.
Fixed MDI/X and fixed speed Typically the PHY performs autonegotiation with the link partner PHY. This means the two PHYs negotiate on the fastest speed and about the MDI connection and polarity. This negotiation takes time, and only after the negotiation the link is established and Ethernet frames are exchange. Configuring the PHY to a fixed configuration speed up the link-up time. PROFINET fast-startup of <500ms

There is a new Ethernet standard adopted to the industrial use case called Single-Pair-Ethernet (SPE). SPE has the advantage of reducing Ethernet cable to one pair over the different Ethernet speed grades of 10Mbps, 100Mbps and 1000Mbps.

PHY SPE Figure 2-8 PHY SPE

The 10BASE-T1L standard supports 10Mbps of bandwidth with a cable reach of 1km (1000m). This is very good because standard Ethernet only supports 100m as referred to above.

100BASE-T1 and 1000BASE-T1 currently support only a cable reach of 50m and 15 m, which is a reduction of maximum cable length of standard Ethernet, but with the benefit of using 1-pair only.