ELM327
Microprocessor Interfaces
A very common question that we receive is ‘Can I
connect the ELM327 directly to my own circuit, or must
I use the RS232 interface shown?’ Certainly you may
connect directly to our ICs, and you do not need to use
an RS232 interface. There are a few items to consider,
however.
The ELM327 is actually a microprocessor that
contains a standard UART type interface, connected to
the RS232 Tx and Rx pins. The logic type is CMOS,
and this is compatible with virtually all 5V TTL and
CMOS circuits, so you should be able to connect
directly to these pins provided that the two devices
share the same power supply (5V), and that they are
not physically more than about 10 to 20 inches apart
(CMOS circuits are subject to latchup from induced
currents, which may be a problem if you have long
leads). If your microprocessor uses a different supply
voltage (for example 3.3V), we recommend that you
look into a commercial interface product, such as the
Texas Instruments TXB0102.
The normal (idle) levels of the ELM327 transmit
and receive pins are at the VDD (5V) level. Most
microprocessors and RS232 interface ICs expect that
to be the idle level, but you should verify it for your
microprocessor before connecting to the ELM327. The
connections are straightforward - transmit connects to
receive, and receive connects to transmit, as shown
below. Don’t forget to set both devices to the same
baud rate.
The ELM327 also provides a hand-shaking feature
that may simplify the flow of data for you. The interface
consists of two pins - an input an an output. The input
is called ‘request to send’ (RTS), and it is used to
interrupt the ELM327, just the same as tapping a key
on the keyboard when using a terminal program. The
output pin (‘Busy’) is used by the ELM327 to tell your
system that it is processing data.
To use the handshaking feature, set one of your
port pins to normally provide a high output, and
connect it to the RTS input (pin 15). Use another port
pin as an input to monitor the ELM327 Busy output
(pin 16). When you want to send a command, simply
check the Busy output first. If it is at a high logic level,
then either wait for it to go low, or if you need to
interrupt the IC, then bring the RTS line low and wait
for the Busy line to go low. (You might want to
consider using an edge triggered interrupt on the Busy
output, if one is available). When Busy does go low,
restore your RTS line to a high level, and then send
your command to the ELM327. No need to worry
about the ELM327 becoming busy again after you
raise the RTS line at this point – once Busy goes low,
the ELM327 will wait (indefinitely) for your command.
If you do not use the RTS input on the ELM327, it
must be connected to a high logic level, as shown
below.
+5V
your microprocessor
+5V
Tx
Rx
L1-L4
+5V
R27-30
470W
+5V
n.c.
16
28
27
26
25
24
23
22
21
20
19
10
18
17
15
The ELM327 and your
microprocessor should
use the same 5V supply
Rx Tx Busy RTS
327
+5V
1
2
3
4
5
6
7
8
9
11
12
13
14
to
R15
to
R21
X1
4.00MHz
C3
27pF
C4
27pF
ELM327DSI
Elm Electronics – Circuits for the Hobbyist
70 of 82
www.elmelectronics.com
ELM [ ELM ELECTRONICS ]
