ELM329LP datasheet(10/87 Pages) ELM

10 / 87 page

Communicating with the ELM329 (continued)

(syntax errors) will always be signalled by a single

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 ELM329 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 ELM329

will continually monitor for either an active RTS input

(if enabled), or an RS232 character received. Either

one can 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 ELM329, 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 ELM329 is not

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

‘AtZ’ are all exactly the same to the ELM329. All

commands may be entered as you prefer, as no one

method is faster or better. The ELM329 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 ELM329 is the ability to

repeat the last 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 obtain an update from the vehicle

- simply send a carriage return character, and the

ELM329 will repeat the command for you. The

memory buffer only remembers the previous command

- there is no provision in the current ELM329 to

provide storage for any more.

10 of 87

ELM329L

ELM329L DSA

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

Please Note:

An issue with the EUSART used in a previous version of the ELM329 resulted in the

possibilty of NULL characters (byte value 00) being inserted into the RS232 data. This

was very rare, but we recommended that software filter for NULL characters and remove

them if found. Although there are no known issues with the ELM329L’s EUSART, we still

recommend that all NULL characters be fitered from data that is received from the

ELM329L.

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