The Evolution of In-vehicle Network Architectures

The drive for fuel-efficient and safer vehicles opened the door to electronic control in vehicles, which in turn led to the deployment of In-vehicle Networks (IVN).  IVNs have become the backbone of modern vehicles. The volume of data flowing through these networks is increasing exponentially with demands for electric vehicles, advanced driver assistance systems (ADAS), radar, lidar, infotainment systems, cameras and vehicle-to-vehicle communications systems.  

To meet this need, the automotive industry—working with technology suppliers—has developed specialized communications protocols and application-specific extensions to existing network technologies, standardized under the aegis of organizations like ISO and IEEE, and it continues to investigate new topologies and protocols to improve performance, increase reliability and lower the costs of IVNs. Two recent developments have filled a longstanding gap in IVN architectures: CAN XL (up-to-20 Mbit/S extended length CAN) and 10Base-T1S (10 Mbit/S single-pair Ethernet), both of which operate in the 10 Mbit/S network space. What problems do these protocols solve and what opportunities do they present for IVN design?

Oscilloscope Testing of 10Base-T1S Automotive Ethernet Signal Integrity

Figure 1. The 10Base-T1S TDME option features
easy eye diagram creation for signal integrity analysis.
Click on any image to enlarge it.

In addition to special serial data bus measurements of 10Base-T1S signals, the 10Base-T1S Trigger, Decode, Measure/Graph & Eye Diagram (TDME) option automates the generation and display of eye diagrams on Teledyne LeCroy oscilloscopes. Eye diagrams are an important element of serial data analysis, used to understand the signal integrity of the communications network. 

The eye diagram is a general-purpose tool for analyzing serial digital communications signals. It shows the effects of additive vertical noise, horizontal jitter, duty cycle distortion, inter-symbol interference, and crosstalk on a serial data stream. 

The eye diagram is formed by overlaying repetitive occurrences of slightly more than a single clock period (UI) of a serial data signal on a persistence display which shows the accumulated history of multiple acquisitions, as shown in Figure 1.

Due to the use of Differential Manchester encoding (DME), the 10Base-T1S eye is formed with twice the signal clock rate. The signal shown has a symbol rate of 12.5 Mbps and the eye is clocked at 25 Mbps. 

Oscilloscope Measurements of 10Base-T1S Automotive Ethernet PLCA Cycle Timing

Figure 1. Color-coded decoding of 10Base-T1S
stream makes it easy to measure timing between
signal elements. Click on any image to enlarge.

The 10Base-T1S Trigger-Decode (TD) and 10Base-T1S Trigger, Decode, Measure/Graph & Eye Diagram (TDME) options enable Teledyne LeCroy oscilloscope users to trigger on and decode Ethernet control and payload data from 10Base-T1S Automotive Ethernet signals. The decoding is color-coded to provide fast, intuitive understanding of the relationship between message frames and other time-synchronous events. Knowing the location of the various protocol elements makes it easy to measure Physical Layer Collision Avoidance (PLCA) cycle timing using either standard oscilloscope tools, or special serial bus measurements included with the TDME options.

PLCA cycle timing is measured to assure interoperability of the attached nodes in a 10Base-T1S mixed-segment, multidrop bus. This class of tests measures the timing between events on the bus relative to a specific bus event, usually the BEACON signal initiated by the Master node. 

Let’s look at a simple example of a 10Base-T1S network with two nodes, the Master (Node 0) and a device (Node 3). The acquired waveform is shown in Figure 2, decoded using the 10Base-T1S TDME option. The top grid shows the complete acquisition, which consists mostly of BEACON signals over a record of twenty-five million samples. Toward the end of the acquisition are two packets from the other nodes. The table at the bottom of the screen lists all the elements decoded in the full acquisition.

Physical-Layer Collision Avoidance in 10Base-T1S Automotive Ethernet

Fig.1, 10Base-T1S PLCA cycle. If there is no data
traffic (top), only BEACONs are seen on the bus. 
Data from a node (bottom) will expand the time
between two BEACONs.

10Base-T1S (IEEE 802.cg) is a variant of Automotive Ethernet  that supports half-duplex and full-duplex communication, allowing either a point-to-point direct connection between two nodes, or use of a multidrop topology with up-to-eight nodes connected on a single 25 m bus segment.

Multidrop cabling of one bus line provides options to extend and scale with fewer physical wires and less weight than point-to-point topologies. With minimum connector space at the ECU, the bus line can be expanded simply by adding sensor units. A bus line with additional sensor units for ultrasonic and short-range radar is an example of how multidrop cabling can be scaled.  

Among the main objectives of the 10Base-T1S PHY layer are reconciliation of transmissions from a variety of mediums, ensuring cooperative behavior by the nodes on a multidrop bus. One way it does this is through the use of Physical-Layer Collision Avoidance (PLCA) technology to minimize dead time and avoid collisions. In this post, we'll describe the workings of PLCA and in a future post, how you can debug PLCA timing issues using an oscilloscope with the 10Base-T1S TDME software.

10Base-T1S Automotive Ethernet vs. 10Base-T1L Industrial Ethernet

Figure 1: 10Base-T1S and 10Base-T1L differ primarily in
reach, encoding methods, topology and applications.

10Base-T1S, a variant of Automotive Ethernet, and 10Base-T1L, also known as Industrial Ethernet, are Single Pair Ethernet (SPE) protocols described in IEEE 802.cg standards. Both offer the same 10 Mb/s communication speed using a single, unshielded twisted pair (T1), but differ in specifics of reach, encoding schemes and topologies, as well as their principal applications.

What Is Differential Manchester Encoding?

Figure 1. Differential Manchester encoding is based on the
presence or absence of a transition, whereas Manchester 
encoding relies on the polarity of the transition.

Differential Manchester Encoding (DME) is an example of a differential, bi-phase encoding technology. DME is specified in the IEEE 802.5 standard for Token Ring local area network (LAN) topology. 

Manchester is categorized as bi-phase encoding because the signal is checked twice every bit interval, also called self-clocking. Each check is one “tick”, each bit interval equals two ticks of the clock. This removes the need for the separate clock signal that is required for Non-Return to Zero (NRZ) encoding. Instead, data and clock signals are combined into a single, two-level, self-synchronizing data stream. The clock can be "extracted" by measuring the timing of the edges.

Figure 2. In Manchester encoding, the polarity of the
transition that occurs mid-interval determines the logic.

In Manchester encoding, we see a digital modulation scheme where voltage transitions rather than voltage levels are used to represent 1s and 0s. In IEEE 802.3 Manchester, a high-to-low transition occurring in the middle of the bit interval represents logical 0, while a low-to-high transition represents logical 1 (the G.E. Thomas variant reverses this logic). Significant transitions always occur in the middle of the bit interval to ensure clock synchronization. Transitions at the start of a period are only used to reset the polarity to achieve the proper transition for the next bit. This increases error detection capabilities, compared to NRZ, but also increases the bandwidth needed to transmit the signal at the same data rate, making it a better candidate for short-distance applications.

Figure 3. Even inverted, DME signals result in same logic.

The most prominent feature that distinguishes DME from classic Manchester encoding is that, in DME only the presence or absence of a transition during the bit interval is important, not the polarity. The presence of a transition represents a logical 0, while the absence of a transition represents a logical 1. Whether the signal goes line-high or line-low depends simply on its state the previous bit interval. This increases bit rate at lower bandwidths, because one bit is guaranteed to occur every interval. It also helps with data recovery in noisy environments, like automotive, because DME allows for a data stream to be inverted, yet still be properly decoded, unlike classic Manchester where the polarity is significant. 

DME is used for 10Base-T1S Automotive Ethernet, a short-distance, low-bandwidth application with either a point-to-point or bus topology up to 25 m. 

Teledyne LeCroy offers QualiPHY compliance test solutions for 10Base-T1S, including a QPHY-10Base-T1-TDR option that automates all required MDI S-parameter tests using the WavePulser 40iX.

For more information about 10Base-T1S compliance testing, see the on-demand webinar, How to Become an Expert in Automotive Ethernet Testing, Part 1.

See also:

Automotive Ethernet MDI S-parameter Testing

Automotive Ethernet in the Vehicle

Fundamentals of Automotive Ethernet

Automotive Ethernet MDI S-parameter Testing

Figure 1: MDI S-parameter tests treat the Base-T1
pair as a balance transmission line and check that
reflections don't cause either excessive power loss
or mode conversion that can disrupt the signal.

As said earlier, the automotive industry has very stringent EMC/EMI requirements, and all Automotive Ethernet standards are designed to ensure good operation even in the presence of high EMI. Not only is there the potential for interference from all the different electronic systems within the vehicle, nothing is stopping you from parking your vehicle below high-voltage transmission wires or in other high EMI fields. 

For this reason, all Automotive Ethernet standards have defined S-parameter tests to be performed at the Medium Dependent Interface (MDI). The assumption is that the single twisted pair that is the basis for all Base-T1 transmissions can be treated as a balanced, differential transmission line with some crosstalk. It is a very real-world application of S-parameters, which can seem so academic.

Two, mixed-mode S-parameters are measured at the MDI reference plane. The tests ensure that there is neither too much loss of power from reflections, nor too much mode conversion into differential signal, that it will disrupt the information of the PAM3 encoded signal.

Automotive Ethernet in the Vehicle

Figure 1 Block diagram of a typical ADAS system
showing the in-vehicle networks used.

As all Automotive Ethernet technologies utilize a single twisted-pair cable (T1) and a point-to-point network topology, they differ primarily in their data rates and encoding methods. For the most part, the data rate determines the applications for which a particular Automotive Ethernet standard can be used. If we take the case of an automatic driver assistance system (ADAS), we can see where each Automotive Ethernet variant might best be used to replace existing automotive protocols. 

(Click on any figure to enlarge the image.)

Fundamentals of Automotive Ethernet

Figure 1: Automotive Ethernet is designed to support
increasingly complex vehicle electronic systems.

When we speak of “Automotive Ethernet”, we’re referring to a group of Ethernet interfaces intended for in-vehicle use, customized to meet the needs of the automotive industry. The first Automotive Ethernet standard was defined by Broadcom in 2011 with BroadR-Reach. Since then, IEEE has released standards for 100Base-T1, 1000Base-T1, 10Base-T1S and most recently MultiGBase-T1. Together, these standards define the general technology known as Automotive Ethernet.

Probably the first question you ask is, “Why not just use standard Ethernet?” A summary of the fundamental features of Automotive Ethernet will show how much better Automotive Ethernet is than standard Ethernet at meeting the industry’s demand for a higher speed, robust, lightweight and lower cost data interface, one that can ultimately replace the many other protocols currently used throughout the vehicle.

Debugging Automotive Ethernet Transmitter Output Droop

Everyone manufacturing devices used for Automotive Ethernet knows what compliance limits they must meet, based on the electrical test specification, but it is not always obvious where to look for the source of the problem when a compliance test fails. We'll provide more Automotive Ethernet debugging tips in future posts, but here is one that arose from a reader's question: namely, why the 45% limit on output droop for 100Base-T1, and what might cause the problem?

Figure 1: Automotive Ethernet electrical testing is performed at the transmitter connector and is largely governed by channel/connector recommendations.  Between the transmitter and the connector is a Low-Pass Filter and a Common Mode Choke both affecting signal droop.