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AN602
Vishay Siliconix
Driver ICs for Automotive Diagnostic Communications Meet ISO 9141 Standards
A series of compact, single-ended bus drivers for automotive diagnostic applications enables communication over the relatively long distances between testing equipment and automotive electronic control units, while protecting sensitive logic devices from the hazards of the automotive environment. The Si9241AEY and Si9243EY are the first narrow-body, small outline products on the market to meet the ISO 9141 standard for this application. Vishay Siliconix ISO 9141 Drivers Vishay Siliconix has produced two drivers to meet ISO 9141 configurations. Each driver is housed in a small-outline surface-mount package with on-board fault protection to protect the controller and tester from the automotive environment. Both drivers have short circuit and over-temperature protection and open load detection. The devices protect against voltage transients ( -3 V 3 [VBAT, K and L] v 45 V), which exceed ISO 9141 (-1 V to 40 V). The Drivers The Si9241AEY (Figure 2) is designed for the user that needs only K-Line communication. The ECU controls the K output through Transmit Pin (Tx) and Chip Select Bar (CS). The fault detection circuitry monitors Tx and K to determine open and shorted loads. If K is high when Tx is low, the K-Line is shorted. Conversely, if K is low when Tx is high, the K-Line is open. Over-temperature protection is always enabled, so a fault is determined whenever the junction temperature exceeds 150_C. All faults are latched until chip select bar is set high. If chip select is not to be used, TX and CS Bar should be tied together, so all faults will reset when TX toggles high.
Connector According to SAE J1978 L K
ISO 9141 (GENERAL DESCRIPTION)
ISO 9141 is an international standard for communications between automobiles and diagnostic testers. It specifies a serial data communication bus between the vehicle's Electronic Control Units (ECUs) and the diagnostic test SAE OBDII Scan Tool (SAE J 1978). ISO 9141 has been adopted by the California Air Resources Board for all cars sold in California with feedback fuel control systems. Similar rules are being adopted in New York, New Jersey, Massachusetts, and Maryland, and are expected to affect the design of most cars sold in the United States.
Diagnostic Tester According to SAE J1978
The Si9243EY (Figure 3) is designed for users that need both K and L Lines for communication. It is packaged in an SO-8 for space savings, but due to pin limitation, it does not have a chip select function. For the same reason, the Fault Detection signal is not bonded out. The fault detection and protection circuitry are active, and the device will shut down in the event of a fault.
VDD VB
ECU 1
ECU 2
Arrows indicate direction of data flow.
RX
FIGURE 1. Possible System Configuration
VDD
- +
VB 2
K CS
Figure 1 is a system configuration for the ISO 9141 specification. The primary form of communication is with the single-ended K-Line. The K-Line passes data bidirectionally, as well as transferring all address information during initialization.
TX Fault Detector FAULT GND
The optional L-Line is unidirectional and is only used to pass address information from the diagnostic tester to the ECUs during initialization. The L-Line is in a 1-state during all other events. The K-Line will mimic the L-Line's address initialization.
Document Number: 70573 16-Jan-01
FIGURE 2. Si9241A K-Line Only Driver
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AN602
Vishay Siliconix
GND VDD VBAT L RXL + L - K RXK + K - + VBAT/2 - K Logic Circuitry (See State Diagram and Truth Table)
The Si924x drivers have an integrated diode in the VBAT line to protect the device from reverse battery conditions. This diode will protect the IC to reverse batteries up to -24 V.
Signal and Communication Specifications
ISO 9141 specifies different logic levels for the Receiver and Transmitter.
VDD TX
Receiver Logic Levels: Logic "0" v 0.3 * VBAT 0.7 * VBAT 3 Logic "1"
FIGURE 3. Si9243A K- and L-Line Driver
Since the fault detection signal is not brought out, it is up to the user to determine if a fault has occurred. One method is to monitor the RxK and TX lines and ensure they are in phase. An exclusive OR can also perform the fault function. The fault condition is cleared when TX is set high.
Transmitter Logic Levels: Logic "0" v 0.2 * VBAT 0.8 * VBAT w Logic "1" The Vishay Siliconix drivers operate to the above specifications, but with a slightly tighter transmission requirement: Logic "1" minimum w 0.91*VBAT. A logic bit transition must be less than 10% of the total bit time. The transition time is measured between the 80% and 20% battery points. Bit time is defined as the time between the 50% battery points of consecutive rising and falling levels.
System Anomalies Two system anomalies in the automotive environment that the Si924x drivers may be subjected to are Reverse Battery and Ground Disconnect.
Regulator VIN VOUT GND 5V VDD RXL Car Battery Micro Processor RXK VBAT/2 K Logic Circuitry K-Line VBAT L-Line Diagnostic Tester VBAT 510 W
VDD TX
Si9243
GND BUS
FIGURE 4. Si9243EY Applications Circuit
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Document Number: 70573 16-Jan-01
AN602
Vishay Siliconix
Physical Layer Figure 5 shows the physical layer capacitance of the ISO 9141 communication scheme. CTE COBW CECU = Tester and Cables = On-Board Wiring = Sum of all ECU Input Capacitance Transmission Before transmitting, the K-Line is a Logic 1 for the time period of the word to be transmitted. An alternate bit pattern is then sent to synchronize the receiver and set the baud rate. After the transmission of the synchronization pattern, two key words are sent to the tester to identify the form of the data and the hardware configuration. When the last key word is sent by the ECU, the tester will echo back the logic inversion of the last word back to the ECU. When this is complete, the ECU will transmit the logic inversion of the initialization address. Conclusion Vishay Siliconix' ISO 9141 bus driver series is produced to save the automotive designer time and space while improving the overall reliability of the diagnostic system. Each driver exceeds the ISO 9141 transient and data requirements, as well saving significant space overall to a discrete solution. With these factors considered, it should reduce manufacturing, building and design costs, and produce a very economical solution to automotive bus interfacing. Reference This application note will only give a general outline of the ISO 9141 communication scheme. The user is advised to read the actual ISO 9141 specification. ISO 9141 - CARB Road Vehicles * Diagnostic Systems. N425/rev. Jan. 91 Initialization Before initialization, the K-Line is a Logic 1 for the time period of the address to be transmitted. The diagnostic tester then sends an 8-bit address on the K and L lines (one start, 8-bit address, one stop) to the ECU.
where CECU + COBW is less or equal to 7.6 nF and CTE is less or equal to 2 nF. The capacitance of the K or L Lines with respect to GND can be no greater than 500 pF. The resistance of the K and L Line is specified with respect to their State.
Logic 1"/Receiving
K (ECU to GND) L (ECU to GND)
Logic 0"
w 50 kW v 110 W w 50 kW N/A
Any resistance placed between the K or L Line and Vb must have a value greater than 100 kW. Data Rate The maximum data rate will vary depending on the number of ECUs and length of cabling. ISO 9141 specifies a maximum bus capacitance of 9.6 nF. Figure 6 shows the maximum data rate versus capacitance. Data Format and Protocol
1000 Vehicle (Clause 9) VB Data Rate (KBaud) Diagnostic Tester (Clause 8) Assumes VBAT above 9 V, and a capacitive load
100
ECU Reading Line K Reading
10
"1" or "0"
I C ECU C ECU C TE
"1" or "0"
1
GND
0.1 1 10 100 Capacitance (nF) 1000 10,000
FIGURE 5. Communication Schematic
FIGURE 6. Maximum NRZ Data Rate with Capacitive Load Si924X with 510-W Load
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Document Number: 70573 16-Jan-01
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