a standard extension cable with the pins
wired 1:1;
not
a null modem cable where the
wires are connected quite differently.
Refer to the parts list for suppliers of the
special components used in this design.
(ELM323 and OBD plug). Hopefully a UK sup-
plier emerges following publication of this
article.
Once everything has been soldered in
place check closely that all the components
are correctly fitted and that there are no sol-
der bridges on the track side. Now the circuit
is complete you are probably anxious
to try it out, but resist the temptation
to plug it into the vehicle connector
without first making some prelimi-
nary tests. Power up the circuit on
the bench using either a 12 V mains
supply or a 9 V battery together with
a computer with a serial interface.
Connect the power supply positive
to pin 16 (+ 12 V) and the negative
to pin 5 (ground). If everything is in
order the green LED will come on to
indicate that the circuit has power),
The red LEDs will then briefly light.
The 5 V supply can now be checked.
When everything is in order the
next step is to connect the vehicle
adapter to the serial port of a PC. It
is now possible to check voltages
around the circuit and compare them
with the typical values given on the
circuit diagram. The +5 V and –0.5 V
shown on pins 6 and 5 of IC1 should
be fairly close to these values but the
voltage on pin 3 of connector K3 is
derived from the PC and depends
largely on the type of interface chip
used in the external computer. It can
be anything from about - 3 V to
- 12 V. The voltage on capacitor C3 is
dependent on the level of this volt-
age and should be about one diode
drop (0.4 to 0.6 V) higher than the
voltage measured at the TxD pin. In
the circuit diagram TxD is assumed
as –9.3 V which then gives -8.9 V
after the voltage drop across D6.
Should you find that the voltage
levels are substantially lower than
the values shown on the diagram
then it is important to find the prob-
lem before we can proceed any fur-
ther. A short circuit on the RS232
interface will usually not result in
damage to the PC because the sig-
nals are current-limited.
For a functional test of the serial
interface we obviously need some
software to run on the PC to send
and receive serial data. The win-
dows program accompanying this
project (to be described in a follow-
up article) would be a suitable tool
for this job but if you are anxious to
TEST
&MEASUREMENT
28
Elektor Electronics
11/2002
(C) ELEKTOR
020138-1
C1
C2
C3
C4
C5
C6
D1
D2
D3
D4
D5
D6
D7
D8
H1
H2
H3
H4
IC1
IC2
K1
K2
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R1
1
R12
R13
R14
T1
T2
T3
T4
X1
020138-1
(C) ELEKTOR
020138-1
Figure 5. The PCB layout (board available ready-made).
COMPONENTS LIST
Resistors:
R1,R3 = 510
Ω
R2,R4 = 2k
Ω
2
R5,R6,R10 = 10k
Ω
R7,R11 = 4k
Ω
7
R8,R9 = 330
Ω
R12 = 47k
Ω
R13 = 100k
Ω
R14 = 680
Ω
Capacitors:
C1,C2 = 27pF
C3-C6 = 100nF
Semiconductors:
D1,D6 = 1N4148
D2-D5 = LED, red
D8 = LED, green
D7 = 1N4001
T1,T2 = BC547B
T3,T4 = BC557B
IC1 = ELM323 *
IC2 = 78L05
Miscellaneous:
K1 = 16-way boxheader
K2 = 9-way sub-D socket (female), angled
pins, PCB mounting
X1 = 3.579545MHz quartz crystal (NTSC),
32pF parallel resonance
16-way OBD-2 plug *
PCB, order code
020138-1
(see Readers
Services page)
* Suggested source for ELM323 and OBD
plug kit:
Küster Datensysteme (KDS)
Geibelstrasse 14
D-30173 Hannover
Germany
Tel. (+49) 511 886059
Fax (+49) 511 8093329
E-Mail:
* OBD connector parts also available from
www.scantool.net
www.autoxray.com
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