ELM327P datasheet(44/51 Pages) ELM

44 / 51 page

A common question we receive with our OBD

interpreters is “Can I connect the ELM32x circuit

directly to my own circuit, or must I use the RS232

interface shown?” Certainly you may connect directly

to our ICs – you do not need to use an RS232

interface.

The ELM327 does not use an inverted RS232 Rx

input (as was the case with the ELM320, ELM322 and

ELM323), so interfacing without RS232 levels is now

even simpler. If you have a microprocessor that uses

the same 5V supply as the ELM327, and has an

internal UART, all you will generally need to do is

connect your processor’s transmit output to the

ELM327’s receive input, and then your receive input to

the ELM327’s transmit output. Communications should

work fine with this connection (and a correct baud rate

setting). The ELM327 is a CMOS device that uses

industry standard levels for all inputs and outputs, so is

easily interfaced to almost any logic circuit.

The ELM327 has a hand-shaking feature that may

be very useful for some interfaces. There is a request

to send (RTS) input, and a Busy output that can be

used to interrupt the ELM327, and to see when it is

Error Messages

BUFFER FULL

The ELM327 provides a 256 byte internal RS232

transmit buffer so that OBD messages can be received

quickly, stored, and sent to the computer or PDA at a

more constant rate. Occasionally (particularly with

CAN systems) the buffer will fill at a faster rate than it

is being ‘emptied.’ Eventually it may become full, and

no more data can be stored (it is lost).

If you are receiving BUFFER FULL messages,

and are using a 9600 baud data rate, give serious

consideration to changing your data rate to 38400

baud. If you still receive BUFFER FULL messages

after that, consider some of the filtering options (the

MR, MT, CF and CM AT Commands).

ready to receive an input. To use them, set one of your

port pins to normally provide a high output, and

connect it to the RTS input. Use another port pin as an

input to monitor the ELM327 Busy output. When you

want to send a command, simply check the Busy

output first. If it is at a logic high level, then bring your

RTS line low and wait for the Busy line to go low (you

might want to consider using an edge triggered

interrupt for this, 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 waits (indefinitely) for your command.

If you do not use the RTS input on the ELM327,

don’t forget to tie it to a high logic level.

Computer Control

BUS BUSY

The ELM327 tried to send the message, or tried to

initialize the bus, and detected too much activity to do

so (it needs a pause in activity in order to insert the

message). Although this could be because the bus

was in fact very busy, it is most often due a wiring

problem that is giving a continuously active input. If

this is an initial trial with your ELM327 circuit, check all

of the voltage levels at the offending OBD input – this

error is very likely due to a wiring problem.

BUS ERROR

A generic problem has occured. This is most often

from an invalid signal being detected (a pulse that is

longer than a Break signal, etc.) on the bus, but could

When hardware or data problems occur, the ELM327

will respond with one of the following short messages.

Some of the messages are suppressed during an

44 of 51

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