ELM322_09 Datasheet, PDF (15/18 Pages) ELM Electronics – OBD (VPW) to RS232 Interpreter

ELM322
Example Applications
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 ELM322 would typically be used. Circuit
power is obtained from the vehicle (OBD pins 16 and
5) and, after some minor filtering, is presented to a five
volt regulator. Notice that the common point of the
regulator is returned to vehicle ground through a diode
and an LED, effectively raising the circuit common
about 2.5 to 3 volts above that of the vehicle. This
gives a net 7.5 to 8 volt positive supply for the OBD
bus, as required by the standard (the ground signal
shown throughout the schematic refers to the circuit
common and not the vehicle’s chassis ground).
Note that by offsetting the regulator in this way,
the LED and the 750Ω resistor (which provides the
current for the LED) become critical components that
must not be eliminated. Also, one other subtle result of
this is that one must take care not to connect the
vehicle’s common to the computer’s common, as the
LED will be shorted out, reducing the supply to 5 volts
which is below the required level.
The remaining connection to the OBD bus (pin 2)
is the data line required for communications. Data is
transmitted onto the bus from the ELM322 via the PNP
transistor, the diode, and the 100Ω current limiting
resistor (which also provides moderate waveshaping).
1
8
9
16
Figure 3. Vehicle Connector
The diode is needed to protect the circuitry from
currents which could flow through the transistor if there
were high voltages on the bus. Note that the 10KΩ
pulldown (loading) resistor returns to vehicle common,
providing the data bus with a full (7.5V) voltage swing.
Data is received from the OBD bus and level
shifted by the NPN transistor shown connected to pin
4 of the ELM322. Using a transistor this way forces the
logic transition point to be at about 3V (the voltage
drops of two diodes and an LED) with respect to
vehicle common. Had the input been directly
connected to pin 4, the threshold would have been
approximately 5 volts - much higher than the 3.5 volts
specified by the standard.
A very basic RS232 interface is shown connected
to pins 5 and 6 of the ELM322. This circuit ‘steals’
power from the host computer in order to provide a full
swing of the RS232 voltages, without the need for a
negative supply. The RS232 pin connections shown
are for a standard 9 pin connector. If you are using the
older 25 pin style, please refer to the web site help
pages for the equivalent pins.
RS232 data from the computer is directly
connected to pin 5 of the ELM322 through a 47KΩ
current limiting resistor. This resistor allows for voltage
swings in excess of the supply levels while preventing
damage to the ELM322. A single 100KΩ 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 to a negative voltage by the computer’s TxD
line). Although it is a simple circuit, it is quite effective
for this type of application.
Finally, the crystal shown connected between pins
2 and 3 is a common television type that can be easily
and inexpensively obtained. The 27pF crystal loading
capacitors shown are typical values only, and you may
have to select other values depending on what is
specified for the crystal you obtain.
This circuit is fully functional and complete as
shown, and has proven itself to also be quite reliable.
Offsetting the two circuit commons may be an
unconventional technique (and one of concern to
many people at first), but it does work very well in
practice.
Many people have written to us saying that their
application requires only one common throughout,
ELM322DSF
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