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ELM329 Datasheet(PDF) 8 Page - ELM Electronics |
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ELM329 Datasheet(HTML) 8 Page - ELM Electronics |
8 / 76 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. 8 of 76 ELM329 ELM329DSB Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com 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 ELM329. Microchip Technology has reported that some ICs which use the same EUSART as in the ELM329 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 ELM329, then monitor incoming bytes, and ignore any that are of value 00 (ie. remove NULLs). |
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