ELM322
Example Applications (cont’d)
might accomplish this. Often, they are interfacing to
other circuits on a vehicle, or wish to connect directly
to a microprocessor, and the different levels that the
commons are at are complicating their design. They
ask for suggestions as to how they might be able to
provide a consistent common throughout.
The circuit of Figure 5 shows one possible way of
doing this. It is slightly more complicated than the one
of Figure 4, and performs just as well.
This circuit uses two voltage regulators in order to
create both the 5V needed for the logic, and the 8V
needed for the vehicle interface. Actually, the J1850
specification says the output level should be between
6.25V and 8V, so when the transistor and diode drops
are taken into account, an 8V supply is ideal. It is
recommended that you use an ‘L’ series regulator for
the OBD output supply (ie. a 78L08 as shown), since it
will limit the short-circuit current to a low level, should
there be problems with the vehicle’s wiring.
While discussing common questions, another one
that is often asked concerns interfacing the ELM322
directly to other logic circuits, without using any
RS232 circuitry. Certainly you can, and are
encouraged to do so. The ELM322 is simply a 5V
CMOS circuit that has plenty of drive capability, and so
can be directly connected to most logic families
without problems. The one difficulty that some people
occasionally encounter is associated with the input
level/polarity at pin 5. The ELM322’s RS232 Rx input
should normally be at VSS (0V) when idle, which is the
opposite to what is usually found with serial (UART)
interfaces. In order to compensate for the ELM322’s
inversion, you need to either make provisions for it in
your software, or else add an external inverter circuit.
If adding and external circuit, it need only be as
simple as the NPN transistor and three resistors
shown for pin 4 (but you might substitute a 10KW
resistor for the 2KWresistor that is shown).
The OBD output circuitry of Figure 5 looks very
similar to that of Figure 4, except that an additional
NPN transistor has been added in series between pin
7 and the PNP transistor. This series NPN transistor is
used to convert the ELM322’s 0V to 5V output swing
to a current which then controls the PNP output
transistor. This configuration (or something similar) is
needed to protect the ELM322 from the 8V, which
would damage it.
The OBD input circuitry shown is very similar to
that of Figure 4. Note that an NPN is again used as the
input device, so that current can only flow into the
receive circuitry, not out of it. If a PNP transistor had
been chosen, its base current might conceivably be
seen as a false active level on some vehicles, which
could tie up the OBD bus, and lead to problems.
The only change between this input circuit, and
the previous one is the addition of a 2.0KW resistor
from the base to the emitter of the NPN transistor. This
resistor works with the 10KWresistor to form a voltage
divider, raising the input threshold voltage to about 3V,
which is almost identical to that provided by the offset
common of Figure 4. A threshold level of about 3V to
to 4V is necessary so that vehicle noise does not
cause false inputs to the ELM322.
We hope that this has helped to provide some
insight and ideas for your design. While both circuits
are fairly simple, the are fully functional and will allow
you to do a great deal with an OBDII equipped vehicle.
As an experimenter, you may want to expand on
these, providing more protection from faults and
electrostatic discharge, or providing
a
different
interface for the RS232 connection to your computer
or PDA. Then perhaps a Basic program to make it
easier to talk to the vehicle, a method to log your
findings, a lookup table for trouble codes, and…
These are the significant differences between the
two interface circuits. There are always many ways of
accomplishing the same thing, and in this case we
have presented two of them for you. They are by no
means the only ways of using the ELM322 - you
should experiment with your own ideas.
ELM322DSE
Elm Electronics – Circuits for the Hobbyist
< http://www.elmelectronics.com/ >
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