ELM329
Example Applications (continued)
on. The following pages show some alternative
communications interfaces that may be of interest if
you are considering modifying the circuit. Also, be sure
to read our Application Note AN04 for a discussion on
connecting with Bluetooth (the ELM327 information
applies to the ELM329 as well).
Figure 11 shows a very basic RS232 interface that
may be connected directly to pins 17 and 18 of the
ELM329. This circuit ‘steals’ power from the host
computer in order to provide a full swing of the RS232
voltages without the need for a negative supply. This
circuit is limited to data rates of about 57.6Kbps, but
has the advantage that it uses common components
and does not require a special integrated circuit. Also,
since it does not exhibit the same inrush currents that
the MAX3222 does, the power supply’s transient
capability is also not of such a concern (you could
likely change C2 to 2.2µF, and C4 to 10µF).
Future Technology Devices (www.ftdichip.com), for
example, produces several. These ICs provide a very
simple and relatively inexpensive way to ‘bridge’
between RS232 and USB, and as you can see, require
very few components to support them.
If using the CP2102, we do caution that it is very
small and difficult to solder by hand, so be prepared
for that. Also, if you provide protection on the data
lines with transient voltage suppressors (TVS’s), be
careful of which ones you choose, as some exhibit a
very large capacitance and will affect the transmission
of the USB data. Note also that the circuit as
presented will operate at a 38400 bits per second rate.
If you want to take full advantage of the speed of the
USB interface, you will need to change PP 0C to
obtain a higher baud rate.
This has provided some examples of how the
ELM329 integrated circuit might be used. Hopefully it
has been enough to get you started on your way to
many more.
The final circuit shows how you might connect the
ELM329 to a USB converter. There are a few single IC
products on the market that allow you to connect an
RS232 system directly to USB. We have shown the
CP2102 by Silicon Laboratories (www.silabs.com) in
Figure 12, but there are others available as well –
Semiconductors
Capacitors (16V or greater, except as noted)
C1 = 0.1uF 50V
D1 = 1N4001
D2 = 1N5232B or SA5.0AG TVS
L1, L2, L3, L4 = Yellow LED
L5 = Red LED
C2 = 10µF 50V
C3, C9, C11, C12 = 0.1uF
C4 = 33µF 10V
L6 = Green LED
C5, C6 = 27pF
U1 = ELM329 (CAN Interpreter)
U2 = MCP2551 (CAN Transceiver)
U3 = LP2950 (5V, 100mA regulator)
U4 = MAX3222E (RS232 Transceiver)
C7, C8 = 560pF 50V
C10 = 0.01µF
C13, C14, C15 = 0.47µF
Misc
Resistors (1/8W or greater)
R1, R2, R3, R4, R5, R6 = 470 W
R7, R8 = 100 W
X1 = 4.000MHz crystal
RS232 Conn = DB9F
ELM329 Socket = 28pin 0.3” (or 2 x 14pin)
R9 = 470 KW
R10 = 100 KW
Figure 10. Parts List for Figure 9
ELM329DSB
Elm Electronics – Circuits for the Hobbyist
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