Elektor EPROM, Manual

Page: 14 / 50

inputs to the ELM323 are protected against
overvoltage while Schmitt triggers reduce the
effects of noise on the inputs.
RS232Tx (Pin 6)
The ELM323 transmits data from this output.
The signal level is compatible with most
interface driver IC’s and there is even enough
current to use a single PNP transistor as a
line driver.
LED Drive outputs (Pin 7, 8, 9 and 10)
These four pins are low when the ELM323 is
transmitting or receiving RS232 or OBD data.
Providing that a suitable series current limit-
ing resistor is fitted, the outputs can source
or sink sufficient current to directly drive an
LED.
OBDIn (Pin 11)
The serial OBD-Data is input on this pin. A
logic high represents the active state of the
OBD K line. There is no Schmitt trigger fitted
to this input so an external input buffer
should be employed to reduce the input sig-
nal transition times.
OBDL (Pin 12) and OBDK (Pin 13)
These active-high output signals are used to
drive the OBD bus using external NPN driver
transistors. Data transfer normally occurs
over the K line but the standards specify that
the driver for the L line be implemented also
to ensure the bus is properly initialised. More
on this later.
V
SS
(Pin 14)
The common ground pin. (The most negative
point in the circuit).
The Interface circuit
The SAE Standard J1962 stipulates
that all OBD compliant vehicles must
provide a standard connector close
to the driver’s seat. The shape and
pinouts of the 16-pin connector has
already been described in the previ-
ous article (Card Diagnosis Systems,
Elektor Electronics October 2002).
The circuit described here plugs
directly into this connector without
any changes necessary to the vehi-
cle.
The male J1962 connector ( Figure
3 ) needed to plug into the vehicles
connector may be difficult to obtain
(see parts list) and you may be
tempted to improvise by connecting
to the back of the vehicle’s OBD con-
nector. If you attempt this, it should
be stressed that you should do noth-
ing to compromise the integrity of
the vehicle’s OBD network. The use
of any type of connector that could
easily short out pins (e.g., the RJ11
type phone connector) is not recom-
mended.
The circuit of the OBD/RS232
interface with the ELM323 is shown
in
Figure 4 . Power is derived from
the vehicle battery (nominally
14.4 V) via pin 16 of the OBD con-
nector (K1) while the vehicle earth is
at pin 5. Voltage regulator IC2 pro-
vides 5 V for the circuit and its built-
in current limit offers some protec-
tion for the circuit. Diode D7 gives
reverse polarity protection. LED D8
‘power’ indicates that the 5 V supply
is available.
TEST &MEASUREMENT
26 Elektor Electronics 11/2002
Block Diagram
2 3 XT1 XT2
0200138 - 12
5
6
Timing and
Control
Interpreter
OBD
Interface
OBDK
11
13
OBDIn
4
OBDL
12
RS232Tx
RS232Rx
LFmode
7
RSRx OBDTx
10
OBDRx
9
RSTx
8
3.58MHz
RS232
Interface
1 8
020138 - 2 - 13
OBD
9 16
Figure 2. Block diagram for the OBD-2/RS232 converter.
Figure 3. The 16-pin connector for the vehicle
diagnostic connector.