ELM327P datasheet(8/76 Pages) ELM

8 / 76 page

Communicating with the ELM327 (continued)

question mark. These include incomplete messages,

incorrect AT commands, or invalid hexadecimal digit

strings, but are not an indication of whether or not the

message was understood by the vehicle. One must

keep in mind that the ELM327 is a protocol interpreter

that makes no attempt to assess the OBD messages

for validity – it only ensures that hexadecimal digits

were received, combined into bytes, then sent out the

OBD port, and it does not know if a message sent to

the vehicle was in error.

While processing OBD commands, the ELM327

will continually monitor for either an active RTS input,

or an RS232 character received. Either one will

interrupt the IC, quickly returning control to the user,

while possibly aborting any initiation, etc. that was in

progress. After generating a signal to interrupt the

ELM327, software should always wait for either the

prompt character (‘>’ or hex 3E), or a low level on the

Busy output before beginning to send the next

command.

Finally, it should be noted that the ELM327 is not

case-sensitive, so the commands ‘ATZ’, ‘atz’, and

‘AtZ’ are all exactly the same to the ELM327. All

commands may be entered as you prefer, as no one

method is faster or better. The ELM327 also ignores

space characters and all control characters (tab, etc.),

so they can be inserted anywhere in the input if that

improves readability.

One other feature of the ELM327 is the ability to

repeat any command (AT or OBD) when only a single

carriage return character is received. If you have sent

a command (for example, 01 0C to obtain the rpm),

you do not have to resend the entire command in

order to resend it to the vehicle - simply send a

carriage return character, and the ELM327 will repeat

the command for you. The memory buffer only

remembers the one command - there is no provision in

the current ELM327 to provide storage for any more.

8 of 76

ELM327

ELM327DSH

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Please Note:

There is a very small chance that NULL characters (byte value 00) may occasionally

be inserted into the RS232 data that is transmitted by the ELM327.

Microchip Technology has reported that some ICs which use the same EUSART as

in the ELM327 may, under very specific (and rare) conditions, insert an extra byte

(always of value 00) into the transmitted data. If you are using a terminal program to view

the data, you should select the ‘hide control characters’ option if it is available, and if you

are writing software for the ELM327, then monitor incoming bytes, and ignore any that

are of value 00 (ie. remove NULLs).

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