Serial Bus System Communication – Introduction

Electronic systems are playing a major role in the development process of new technologies and products. In every major industry area (energy, transportation, civil engineering, etc.) electronic systems are used more and more. This process of using heavily electronic systems is called electronification.

If we take as example the automotive industry we can see that electronic systems are introduced in a greater and greater number. This is due to increasingly more stringent exhaust emissions legislation and customers demand for more safety and comfort. Another driver factor for using electronic systems is the reduction of time-to-market for new models, cost reduction and higher competition between vehicle manufacturers.

A vehicle is made up from several system interconnected to each other. These systems are:

  • engine
  • automatic transmission (AT)
  • heating, ventilation and air conditioning (HVAC)
  • steering
  • braking
  • navigation
  • etc.

Each of these systems are controlled by an Electronic Control Unit (ECU). An ECU can be regarded as a miniaturized computer which reads inputs from sensors (temperature, speed, pressure, etc.), the data is used by an internal decision logic, which ultimately controls some actuators (electric motors, valves, etc.).

When the process of electronification started, every ECU was working independently of the others. After a short period it was discovered that, if the ECUs exchange information between themselves, the overall performance of the vehicle can be improved in term of emissions, driveability, comfort and safety.

First step in connecting the ECUs between themselves was to use a wire for each electric signal (information).

Conventional Networking Layout for ECUs Communication

Image: Conventional Networking Layout for ECUs Communication

This method of connectic several electronic controllers between themselves was proved to have a lot of drawbacks:

  • expensive, due to the numbers of wires and connections
  • not reliable, due to high probability of failure
  • heavy, the number of wires were adding substantial weight to the vehicle

To overcome all these problems a bus system communication approach was used. This means that all the ECUs use the same wire(s) to communicate with each other. Of course, this is possible only with the implementation of a communication protocol which has to convert the data in a specific format (at the sender side) and decode it in the same way (at the receiver side).

Bus Networking Layout for ECUs Communication

Image: Bus Networking Layout for ECUs Communication

For example (above), we have four ECUs connected to the same bus (wire). All of them have to use the same protocol for communication. A protocol can be regarded as a set of rules which are used to send and receive electronic signals.

A very simple example could be a Bit Serial Data Transmission protocol. We know that a decimal number can be converted into a binary number. A binary value could be either 0 or 1. If we assume that 0 means 0 V and 1 means 12 V, transmitting at a fixed frequencies, either 0 V or 12 V we can encode our decimal values.

Example: The engine (ECU-1) needs to transmit the value of the mean effective torque (75 Nm) to the automatic transmission (ECU-2). In this case the engine is the transmitter and the automatic transmission is the receiver.

We convert 75 in binary:

\[75_{10} = 01001010_{2}\]

ECU-1 will send a signal (voltage changes) across the bus, either 0 V or 12 V, to match the binary value of the torque.

Bit Serial Data Transmission Example

Image: Bit Serial Data Transmission Example

Since both ECUs follow the same protocol, ECU-2 knows that it needs to receive 8 bits. At regular intervals, ECU-2 will measure the voltage of the bus and will assign a 0 for 0 V and a 1 for 12 V. After reading eight bits it will convert the binary number to decimal and get the value of the torque.

There are a lot of things to consider for the protocol, in order for the data transmission and reception to work properly:

  • data identification
  • synchronization
  • data check
  • etc.

Compared to conventional wired network, bus systems offer significant advantages:

  • lower cost of the material for the wires
  • lower space and weight for the installation of the wires
  • lower number of connections for every ECU
  • data can be distributed between several receivers

The serial bus communication system can be regarded as an introductory technology for more advanced protocols: CAN, LIN, FlexRay, etc.

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