10BaseT1S Protocol

Overview of the Protocol and Decoding using Tektronix Oscilloscope and application software from Prodigy Technovations.

An ubiquitous Ethernet architecture simplifies the design, configuration, and control of many different applications, whether in the industrial connectivity or  in the computers. Now it is the turn for the automotive industry.

From last few years automotive industry is adopting 100BaseT1 and 1000baseT1 ethernet technology to address the high-speed data rate need in the in-vehicle network.  Now slowly automotive industries are trying to reduce the use of different serial bus technologies and shift to one most adaptable technology which is an ethernet variant and then can also meet most needs of an in-vehicle network.

Traditionally automotive industry used CAN1.0/2.0, Can FD, FlexRay, LIN, MOST, D2B, Byteflight serial buses to address different needs in an in-vehicle network.  So many technologies interconnected resulted in a heterogeneous network.

In order to address the high-speed data rate, automotive industry adopted 100BaseT1 full duplex 100 Mbps automotive ethernet to start with by updating the firmware of the vehicle. Slowly, 100BaseT1 technology was being used for more applications within the in-vehicle network. Now, IEEE has also announced 1000baseT1 and 10BaseT1S to address the high and low speed networks of the in-vehicle network. It is expected that 10BaseT1S is progressively going to  replace the applications addressed by traditionally used serial bus technologies based on the mission and safety critical needs. In an automotive in-vehicle network majority of the protocol traffic speed is less than 10Mbps. Hence 10baseT1 can be the most adaptable  serial technology to address this need.

10BaseT1S is a 10Mbps, single pair twisted wire, short range ethernet bus. It is a multidrop technology which makes it very different than any other existing ethernet technologies. It is designed to support stringent needs of the electromagnetic compatibility needs for the  automotive industry.

 

Fig. 1 10BaseT1 Bus (Typical)

 

10baseT1S uses single twisted pair cabling like CAN and FlexRay Bus.  All ECUs (nodes) with 10BaseT1 are connected to the bus and protocol packet collision is avoided using Physical layer collision avoidance (PLCA).   Access to bus for each of the 10BaseT1S ECU is deterministic. When one node is transmitting the data other nodes are in high impedance state and are in the listen mode.

10BaseT1 is a differential signaling 1V peak to peak with 20% design margin. Each data is encoded using Differential Manchester Encoding (DME).

Fig. 2 Physical layer encoding of bits using DME

Clock transition helps all nodes to synchronize. If there is transition between the clock transition it is considered as logic 1. If there is no transition between the clock transition it is considered as logic 0. Normally data is encoded using voltage levels in all other ethernet standards but in 10BaseT1s, data is encoded using clock transitions. This provides better EMI shielding.

In the physical layer data received from MII (Media-Independent Interface) is scrambled and then 4b/5b encoding is done as per bi- phase Manchester encoding method. Some of the 5b encoded symbols are used for aligning the pattern to maintain and control the network.

Let us look at the method used for collision avoidance of the data in 10BaseT1s. 10BaseT1S implements Physical Layer Collision Avoidance (PLCA). To support this method one of the ECU will be assigned ID ‘0’ (Zero). This ECU with node ID ‘0’ will co-ordinate the transmission cycle. ECU with node 0 will transmit a beacon which informs all the nodes on start of the transmission cycle. Then node 0 has the top priority to transmit the data over the bus. Node 0 must send commit within 20 bits from the beacon if it has a data to send, if not node 1 will get the time window to send the data by sending commit. Please note maximum data symbols supported by the data frame is 1500 byte long. If more than 1500 data symbols are to be transmitted, then 10BaseT1S uses burst mode. This process will repeat till all the nodes get the time window to send the data on the bus. Then master ECU ‘0’ will send the beacon to indicate to start for the next transmission cycle. By following this round robin transmission cycle, the data transmission process of 10BaseT1S implements PLCA.

Fig. 3 Ethernet packets over the 10baseT1S bus

 

In Fig. 3 we can observe that after the Beacon, ECU 0 does not having any data to send. Hence it is not participating by sending commit. Whereas ECU1 has the data. Hence it will send the data by sending commit followed by ethernet with maximum of 1500 data symbols. If there is more than 1500 data, it can use burst mode for data transmission.

Each transmission cycle can be of different length for a 10BaseT1S bus, since only some nodes may transit the data, some may not, and some nodes may even have more data to send. And when there is more data to be sent then the ECU must send data using the burst mode.

 

Overview of 10baseT1S Protocol

Four bits of data is encoded as 5b in 10BaseT1S ethernet PHY.  Following 5b symbols are used for maintaining and controlling the operation of the 10baseT1S bus.

10BaseT1S has adopted 802.3 ethernet frame as per below:

Fig. 4 10BaseT1S Ethernet Frame

The above frame adoption makes 10BaseT1S ethernet frame, non-compatible with other ethernet standards in the physical layer. 10baseT1S frame has following packets contents

Commit has JJHH  5b symbols, 20 bits

Descrambler Initializer is transmitted so that receiver node in the 10BaseT1S bus can decode the data frame.

IEEE preamble is used to synchronize the clock to the transmitter.

Frame Sequence Check or CRC  is optional for non-MII interface. In case of non-MII interface, such designs may implement different techniques for error checking methods. In such case FSC is redundant and only occupies in the bandwidth.

 

Debugging 10baseT1S bus using PGY-10BaseT1S Protocol Decode Software

While designing 10BaseT1S automotive ethernet bus, there is a need to test the protocol operation of the 10baseT1S Bus. Oscilloscope based application software provides the convenient method of decoding the solution.

PGY-10BaseT1S Protocol Decode software runs inside windows operating system-based oscilloscope from Tektronix. It can read the live or saved data from the oscilloscope and can decode the 10BaseT1S acquired waveform. The result will display the timestamp, 10BaseT1 ethernet frame details and checks data from FCS pass/fail condition.

For effective debugging purpose, software provides over lay of decoded packet next to the waveform in the selected frame view.

By selecting the 10BaseT1S frame in the listing window user can directly view the corresponding ethernet waveform frame.

The decoded data can be exported to CSV, txt files for further debug and analysis. Also, html report can be generated for internal documentation purpose.

For more info: PGY-10BaseT1S Protocol Decode software

 

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