Vehicle Diagnostics Adapter (1)

Interface between an OBD-2 vehicle diagnostic connector and a serial PC port

In last month’s article we took a look at the background and specification of the OBD-2 diagnostic connector fitted to new cars. Now, as promised we present an interface adapter that allows your car to confess its innermost secrets to your computer.We can thank an EU directive dating from 1998 for the introduction and standardisation of the OBD-2 vehicle diagnostic connector. The connector is standard on all spark-ignition engined vehicles from 2000 and should be fitted to diesel engined variants by 2003. The communication protocols used can come in three different varieties. In Europe the most common protocol is the ISO standard. All of these protocols rely on serial data transfer but the signal levels and message format are not compatible with the serial communications port of a Personal Computer.

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Materials

020138-PCB.pdf

Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor GREEN and GOLD members can exclusively download these files for free as part of their membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.

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PCB

Download Gerber file: 020138-1.zip

Component list

Resistors:
R1,R3 = 510?
R2,R4 = 2k?2
R5,R6,R10 = 10k?
R7,R11 = 4k?7
R8,R9 = 330?
R12 = 47k?
R13 = 100k?
R14 = 680?
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
* 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-Service@KDS-Online.com
* OBD connector parts also available from www.scantool.net and www.autoxray.com