PSI5 (Peripheral Sensor Interface 5) designates a sensor bus system that is used mostly with a lower data transfer rate below 200 kbps. PSI5 was developed by the companies Autoliv, Bosch, and Continental and is based on the already existing PAS4 protocol. A secure and faster data transfer is guaranteed with reduced system expenses. The applications with the PSI5 bus system contain up to four sensors per bus node. PSI5 is structured by a two-wire line and is used mainly in automobile electronics. Here, sensors are coupled to electronic control units, hereinafter called ECUs. Point-to-point connections as well as bus configuration with asynchronous or synchronous communication can be realized.
Areas of Application of the PSI5
The areas of application are possible wherever a low data transfer rate is used. Here, use of the PSI5 is considered as more secure and faster. The data transmission is not impaired or infringed even through the EMC factors and this is a clear advantage over analog connections. PSI5 is also used in the field of bidirectional communication. The typical applications include primarily all airbag applications with pressure sensors in the automobile sector. However, the bus system can be found in automobiles not only in the airbag, but also in angle measuring (for example in the accelerator pedal), as well as in acceleration sensors
Synchronous and asynchronous data transfer
The data exchange can take place synchronously or asynchronously.
- Synchronous Data Transfer
The time multiplex process is used for synchronous transfer of the data. With this, the ECU sends synchronization pulses, and the actuators and sensors are allotted specific time spans for the data transfer.
- Asynchronous Data Transfer
The asynchronous data transfer is performed without synchronization pulses of the ECU. The sensors or actuators automatically send their data in fixed periodic intervals to the control unit.
Different bus topologies
The different options for data transfer also result in different variations of the topology for a network. A star or line topology is suitable for synchronous operation. However, the conventional point-to-point connection is used for asynchronous operation. The line topology is also used in daisy-chain operation. With this, the sensors and actuators are initially coupled in series. However, after assignment of the time spans for transfer of the data packets, they switch to parallel mode.
Flow of the data transfer
Not only the time spans are assigned to the sensors or actuators when the synchronization pulses of the ECU are sent. Configuration data or diagnosis enquiries are also sent at the same time. The Manchester coding ensures that there is no signal interference during the transfer. This function of the Manchester coding is also supported by the structure of the data words. These are composed of two start bits each, 8 to 24 data bits, and a parity bit. This provides a high interference resistance. The data transfer rates are between 125 to 189 kbps and thus clearly higher than with LIN connections, although below the transfer rates of the CAN buses, whereby the wiring expense is higher with CAN.
Today, PSI5 is an open standard. The first joint publication of the PSI5 by the three main participating companies in the development took place in May 2005 at the sensor fair in Nuremberg. With this step, the PSI5 was standardized in the products of the companies Autoliv, Bosch, and Continental. Other companies specifying the bus system as standard in their products were added in 2006 and 2007. In 2006, Siemens VDO declared their support for the standardization, and Freescale and TRW followed in 2007.
Expansion by ELMOS Chip
A compatible transfer system with PSI5 can be built with the chip 910.85 from ELMOS and a micro controller. With this, 4 channels with 3 sensors each can be operated, all operating independent of each other. The ELMOS product has an integrated chip that enables the functions of pressure acquisition, filtering, Manchester decoding, and current modulation. This system is used mainly for airbag applications and pressure measuring applications.
The following listing provides an overview of all functions of the transfer system:
- Four operation channels, operating independent of each other, each equipped with 3 sensors.
- Furthermore, the line topology, the conventional point-to-point connections, and daisy-chain operation can be used.
- A protection function against 40 V is installed for each channel.
- Adaptive control of the data comparator threshold is possible.
- All transmitted data is encoded with the Manchester code.
- The typical operation voltage is 5 and 3.3 V.
- An SPI interface is provided.
- Each channel has a built-in overvoltage and overtemperature function.