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
[V
BAT
, 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.
V
DD
V
B
ECU 1
ECU 2
Arrows indicate direction
of data flow.
RX
FIGURE 1.
Possible System Configuration
V
DD
–
+
V
B
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
www.vishay.com
1
AN602
Vishay Siliconix
GND
V
DD
V
BAT
L
RXL
+
L
–
K
RXK
+
K
–
+
V
BAT
/2
–
K
Logic Circuitry
(See State Diagram
and Truth Table)
The Si924x drivers have an integrated diode in the V
BAT
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.
V
DD
TX
Receiver Logic Levels:
Logic “0”
v
0.3 * V
BAT
0.7 * V
BAT 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 * V
BAT
0.8 *
V
BAT
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*V
BAT
.
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
V
IN
V
OUT
GND
5V
V
DD
RXL
Car
Battery
Micro
Processor
RXK
V
BAT
/2
K
Logic Circuitry
K-Line
V
BAT
L-Line
Diagnostic Tester
V
BAT
510
W
V
DD
TX
Si9243
GND
BUS
FIGURE 4.
Si9243EY Applications Circuit
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2
Document Number: 70573
16-Jan-01
AN602
Vishay Siliconix
Physical Layer
Figure 5 shows the physical layer capacitance of the ISO 9141
communication scheme.
C
TE
C
OBW
C
ECU
= 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 C
ECU
+ C
OBW
is less or equal to 7.6 nF and C
TE
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 V
b
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)
V
B
Data Rate (KBaud)
Diagnostic Tester
(Clause 8)
Assumes V
BAT
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
www.vishay.com
Document Number: 70573
16-Jan-01
3
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