Elektor EPROM, Manual

Page: 15 / 50

of data flowing at both the OBD and RS232
interface. The two groups of LEDs share a
common current limiting resistor because
data will only be flowing in one direction at
any one time on either of the two interfaces
(the ELM323 is not capable of true multitask-
ing). The OBD bus may also be in its initiali-
sation phase when RS232 data is sent to it so
these limiting resistors are separate for the
OBD and RS232 interface.
A crystal is fitted between pins 2 and 3 of
IC1 along with two 27 pF loading capacitors.
The capacitor values shown are typical but
these may need to be changed depending on
the crystal specification. The frequency cho-
sen is that of the NTSC standard TV colour
burst crystal and should be relatively cheap
and widely available.
Construction and test
The layout for the circuit can be seen in Fig-
ure 4 . Although the PCB is single-sided it
does not need any wire links to be fitted. Con-
nector K2 is a 9-way sub-D
socket (do not
make the mistake of fitting a plug).
As for the RS232 cable make sure that it is
The remaining two connections to
the vehicle (OBD Pin 7 and 15) are
the data lines described in the ISO
9141 and ISO 14230 standards. In
accordance to the standards pin 7 of
the connector is the K output while
pin 15 is the L output. We will refer
to these as the K line and L line of
the OBD system. To comply with the
specification, the ELM323 controls
these two lines using NPN transis-
tors with 510
Ω
pull-up resistors.
The adapter circuit receives diag-
nostic data from the K line (pin 7 on
the OBD connector). The data is
inverted by transistor T3 before it is
read by IC1 (pin 11). This transistor
stage raises the threshold voltage to
around 4 V instead of the standard
2.5 V for a CMOS input. The effect of
this is to improve noise immunity on
the input and the stage gain speeds
up signal transition times.
For the interface to a computer
there is a very simple RS232 imple-
mentation using just RxD (Pin 2) and
TxD (Pin3) of the 9-pin sub-D con-
nector. Most RS232 interface circuits
require a voltage converter to pro-
duce a negative supply to allow the
correct signal swing for RS232 sig-
nals. This design however, stores
negative charge from the TxD line
onto capacitor C3 to ensure that data
output from the ELM323 will swing
negative when T4 switches off.
Resistor R12 limits the input current
from the computer. R13 ensures that
the RS232 input (IC1, pin 5) will be
pulled low when the connector at K2
is disconnected. Transistor T4 drives
RS232 data to the PC. The signal
voltage will swing b5 V
(high) when T4 is conducting to
–5.1 V (low) from the negative
charge stored on C3 when T4 is off.
Despite the simplicity of this RS232
interface it works very well.
Pin 4 of IC1 is tied high to force
the microcontroller to send a line
feed (LF) character after each car-
riage return (CR) character.
The four LEDs connected to pins
7, 8, 9 and 10 give a visual indication
TEST &MEASUREMENT
27 11/2002 Elektor Electronics
+5V
X1
3.579545MHz
C1
27p
C2
27p
K2
DB9
1
2
3
4
5
6
7
8
9
D4 D5 D3 D2
R8
330
Ω
R9
330
Ω
C6
100n
C4
100n
C5
100n
C3
100n
R10
10k
R12
47k
R2
2k2
R4
2k2
R6
10k
R1
510
Ω
R3
510
Ω
R5
10k
R7
4k7
R11
4k7
R13
100k
T3
BC557B
T4
BC557B
T1
BC547B
T2
D6
1N4148
D1
1N4148
RS232Rx
RS232Tx
ELM323
LFmode
OBDIn
OBDRx
OBDTx
IC1
RSRx
RSTx
OBDL
OBDK
XT2 XT1
11
14
10
12
13
2 3
1
5
4
6
7
8
9
78L05
IC2
D8
R14
680
Ω
POWER
+5V
D7
1N4001
K1
10
11 12
13 14
15 16
1 2
3 4
5 6
7 8
9
2x
020138 - 11
OBD
(ISO)
RS232
5V
– 5V1
– 0V5
– 9V3
– 8V9
Figure 4. The OBD-2 to RS232 adapter circuit diagram.