ELM320
Example Application
The SAE J1962 standard dictates that all OBD
compliant vehicles must provide a standard connector
near the driver’s seat, the shape and pinout of which is
shown in Figure 3 below. The circuitry described here
will be used to connect to this plug without modification
to your vehicle.
The male J1962 connector required to mate with a
vehicle’s connector may be difficult to obtain in some
locations, and you could be tempted to improvise by
making your own connections to the back of your
vehicle’s connector. If doing so, we recommend that
you do nothing which would compromise the integrity
of your vehicle’s OBD network. The use of any
connector which could easily short pins (such as an
RJ11 type telephone connector) would definitely not
be recommended.
The circuit of Figure 4 on the next page shows
how the ELM320 would typically be used. Circuit
power has been obtained from the vehicle (via OBD
pins 16 and 5) and, after some minor filtering, is
presented to a low power (100 mA) 5 volt regulator.
The output of this regulator powers several points in
the circuit as well as an LED (for visual confirmation
that power is present).
The remaining two connections to the vehicle
(OBD pins 2 and 10) are for the differential data
system specified by the J1850 PWM standard. When
no data is being transmitted, both wires are idle with
the transistor drivers off, and the resistive pullup and
pulldown allow voltage levels to float to the supply
levels. Note that the PNP driver transistor and the
2.7KW pullup resistor both have series protection
diodes to prevent backfeeds into the ELM320 circuitry.
The ELM320 has only one OBD data output line
(pin 7). It is an active low signal, so must be used to
drive the open-collector ‘Bus +’ signal via the PNP
transistor as shown. By using a portion of this same
signal to drive the NPN transistor for the ‘Bus -’ signal,
one obtains open collector differential drive.
negative supply. The RS232 pin connections shown
are for a 25 pin connector. If you are using a 9 pin, the
connections would be 2(RxD), 5(SG) and 3(TxD).
RS232 data from the computer is directly
connected to pin 5 of the IC through only a 47KW
current limiting resistor. This resistor allows for voltage
swings in excess of the supply levels while preventing
damage to the ELM320. A single 100KW resistor is
also shown in this circuit so that pin 5 is not left floating
if the computer is disconnected.
Transmission of RS232 data is via the single PNP
transistor connected to pin 6. This transistor allows the
output voltage to swing between +5V and the negative
voltage stored on the 0.1µF capacitor (which is
charged by the computer’s TxD line). Although it is a
simple connection, it is quite effective for this type of
application.
Finally, the crystal shown connected between pins
2 and 3 is a common TV type that can be easily and
inexpensively obtained. The 27pF crystal loading
capacitors shown are only typical, so you may have to
select other values depending on what is specified for
the crystal you obtain.
This completes the description of the circuit. While
it is the minimum required to talk to an OBD equipped
vehicle (it relies on such techniques as using the
internal current limiting of the 78L05 for circuit
protection, for example), it is a fully functional circuit.
As an experimenter, you may want to expand on it,
though, providing more protection from faults and
electrostatic discharge, or providing
a
different
interface for the RS232 connection to the computer.
Then perhaps a Basic program to make it easier to talk
to the vehicle, and a method to log your findings,
and…
1
9
8
Data is received from the OBD bus, then conerted
and level-shifted by the NPN/PNP transistor pair that
are shown connected to pin 4 of the ELM320. The
NPN transistor detects the differential data signal while
allowing for the presence of common mode voltages,
and the PNP transistor provides the 0 to 5 volt levels
required by OBDIn.
16
Figure 3. Vehicle Connector
A very basic RS232 interface is shown connected
to pins 5 and 6 of the ELM320. This circuit ‘steals’
power from the host computer in order to provide a full
swing of the RS232 voltages without the need for a
ELM320DSC
Elm Electronics – Circuits for the Hobbyist
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