PSI5 - 1705

Overview
The Peripheral Sensor Interface 5 (PSI5) is a digital interface published by the PSI Consortium ([6]) for sensor applications in motor vehicles and can be assigned by the application to Class A (see Table 1). The PSI5 is based on already existing interfaces for peripheral airbag sensors, but has been developed as an open standard which can be used and implemented at no extra charge. The technical characteristics mentioned in the following, the low implementation expenditure and the low additional costs compared with an analog sensor connection make the PSI5 an
attractive option for sensor applications in motor vehicles too.

 

Transfer system
The PSI5 is a two-wire current interface in which the same lines are used to power the sensors and for Manchester-encoded data transfer. The bus master in the ECU modulates a voltage to the sensor for this purpose. Data transfers from the sensor to the ECU are effected by current modulation of the supply lines. In this way, high EMC robustness and low electromagnetic radiation are achieved. A wide range of supply currents for the sensors can be supported.

The different PSI5 operating modes define the topology and parameters of communication between ECU and sensors
(Figure 12).

– Communication modes: The asynchronous mode can be used for a unidirectional point-to-point connection. In the
three synchronous bus modes (parallel, universal or daisy-chain cabling) several sensors can communicate on
a time-controlled basis with the bus master bidirectionally and using TDMA processes.
– Data-word width: The PSI5 supports avariable data-word width of 8, 10, 16, 20 or 24 bits.
– Error detection: This can be effected either with a parity bit with even parity or with three CRC checksum bits.
– Cycle times: These are specified in μs.
– Number of time slots per cycle.
– Data-transfer rate: As standard 125 kbit/s or optionally 189 kbit/s

 

 For example, the operating mode “PSI5-P10P-500/3L” denotes a parallel synchronous mode with ten bits per data word and parity bit for error detection. The data are transmitted every 500 μs with three time slots per cycle and a low bit rate.

During communication from sensor to ECU, a “low level” is represented by a normal (non-oscillating) current input of
the sensor. A “high level” is generated by an increased current sink of the sensor (typically 26 mA). This current modulation is detected by the receiver in the ECU. Each PSI5 data packet consists of N bits, in which there are in each case two start bits and one parity bit (or three CRC bits) and N − 3 (or N − 5) data bits. The data bits are transmitted with the least significant bit (LSB) first. Error detection with a parity bit is recommended for eight or ten bits, three CRC bits for longer data words. In a PSI5 message, the data and value ranges have different meanings. One range serves to transmit the sensor output signals (≈ 94 %), one range the status and error messages (≈ 3 %), and one range the initialization data (≈ 3 %).

 After each starting or undervoltage reset, the sensor conducts an internal initialization, after which the sensor is in an
executable mode. Whereas communication by the sensor with the ECU is effected with current signals, voltage modulation of the supply lines is used during communication by the ECU with the sensors. A logic 1 is represented by the synchronization signal, a logic 0 by the absence of the expected synchronization signal within the designated time slot. The synchronization signal consists of four voltage phases (Figure 13):
– Sync Start (nom. 3 μs, < 0.5 V),
– Sync Slope (nom. 7 μs, > 0.5 V),
– Sync Sustain (nom. 9 μs, > 3.5 V), and
– Sync Discharge (nom. 19 μs, < 3.5 V).