ELM327LSM datasheet(77/94 Pages) ELM

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A very common question that we receive is ‘Can I

connect the ELM327 directly to my own circuit, or must

I use the interface shown?’ Certainly you may connect

directly to our ICs, and you do not need to use an

RS232 or USB 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 the Rx pin has Schmitt trigger waveshaping, so

you should be able to connect directly to these pins

provided that the two devices share the same power

supply (VDD), and that they are not physically more

than about 10 to 20 inches apart (CMOS circuits are

subject to latch-up from induced currents, which may

be a problem if you have long leads).

The normal (idle) levels of the ELM327 transmit

and receive pins are at the VDD (logic high) 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. Note that

the default setting for PP 0E turns these hand-shaking

Microprocessor Interfaces

+5V

4.00MHz

27pF

R27-30

470

Ω

+5V

27pF

+5V

R15

R21

n.c.

+5V

327

L1-L4

your microprocessor

The ELM327 and your

microprocessor should

use the same 5V supply

Busy RTS

77 of 94

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Elm Electronics – Circuits for the Hobbyist

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