ELM327
Example Applications (continued)
what you are doing while testing.
been provided as a visual means of confirming circuit
activity. They are not essential, but it is nice to see the
visual feedback.
The final OBD interface shown is for the two
J1850 standards. The J1850 VPW standard needs a
positive supply of up to 8V while the J1850 PWM
needs 5V, so we have shown a two level supply that
can provide both. This dual voltage supply uses a
317L adjustable regulator as shown, controlled by the
pin 3 output. With the resistor values given, the
selected voltages will be about 8.0V and 5.5V, which
works well for most vehicles. The two J1850 outputs
are driven by the Q1-Q2 combination for the Bus+,
and Q3 for the Bus-.
The J1850 VPW input uses a resistor divider,
similar to that which was used for the ISO input.
Typical threshold voltages with the resistors shown will
be about 4.2V (rising) and 2.2V (falling). The J1850
PWM input is a little different in that it must convert a
differential input into a single-ended one for use by the
ELM327. This is done by connecting Q4 across the
input so that it operates as a difference amplifier. The
Q4-D3 series combination sets a threshold voltage of
about 1V (for improved noise immunity), while R11
limits the current flow, and R12 makes sure that Q4 is
off when it should be. The circuit works well as shown,
but the R14 passive pullup time constant can be easily
affected by stray capacitance - be aware of this when
connecting test equipment to the PWM input pin.
Resistor R1 is the final J1850 component. We
added this to help discharge the data bus more rapidly
when it was found that some vehicles showed higher
capacitance than others. The resistor may not be
required at all for many vehicles - the choice is yours.
If you should see reports of BUS ERRORs with a
J1850 vehicle, it may be this capacitance causing
problems (you will need to ‘scope the signal to be
sure).
On the right side of the circuit, the ELM327’s
RS232 pins (17 and 18) are shown connected to an
FTDI USB module. This module makes it very easy to
connect the ELM327 circuit to your computer - all you
need is the VCP Driver software, which is available for
free from the FTDI web site (www.ftdichip.com). The
module pinout matches a 9 pin D-sub connector, so
you can simply solder it in where the RS232 circuitry
used to go. Diode D5 and resistor R32 have been
added to the interface to prevent backfeeds from the
USB supply into the ELM327.
Finally, the crystal shown connected between pins
9 and 10 is a standard 4.000MHz microprocessor type
crystal. The 27pF crystal loading capacitors shown are
typical only, and you may have to select other values
depending on what is specified for the crystal that you
use. The crystal frequency is critical to circuit operation
and must not be altered.
We often receive requests for parts lists to
accompany our Example Applications circuits. Since
this circuit is more complex than most, we have named
and numbered all of the components and provided a
summary parts list (Figure 10). Note that these are
only suggestions for parts. If you prefer another LED
colour, or have a different general purpose transistor
on hand, etc., by all means make the change. A quick
tip for those having trouble finding a 0.3’ wide socket
for the ELM327: many of the standard 14 pin sockets
can be placed end-to-end to form one 0.3’ wide 28 pin
socket. For more help with building and testing the
circuit, see our ‘AN02 - ELM327 Circuit Construction’
application note.
What if you only want to support one of the
protocols? Well, you need to be careful with the
unused pins, but other than that, the connections are
straight-forward. Figure 11 shows how you might
support only the SAE J1850 VPW protocol.
Compare the OBD protocol portions of the circuits
in Figures 9 and 11 - the differences should be very
apparent. The unused protocols in Figure 11 have
simply had their outputs ignored (left open circuit), and
their inputs wired directly to high or low logic levels.
Which level to connect them to is shown in the
‘Unused Pins’ section on page 5. Note that unused
CMOS inputs must always be connected to either a
high or a low level - they should never be left floating.
The circuit of Figure 11 maintains the status LEDs,
The R25-R26 voltage divider shown connected to
pin 2 is used for the vehicle voltage monitoring
circuitry. The two resistors simply divide the battery
voltage to a safer level for the ELM327, and the
capacitor (C2) helps to filter out noise. As shipped, the
ELM327 expects a resistor divider ratio as shown, and
sets nominal calibration constants assuming that. If
your application needs a different range of values,
simply choose your resistor values to maintain the
input within the ELM327’s VSS to VDD voltage range,
and then perform an AT CV to calibrate the ELM327 to
your new ratio. The maximum voltage that the ELM327
can show is 99.9V (it’s a software limit, not hardware).
The four LEDs shown (on pins 25 to 28) have
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