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ELM322SM Datasheet(PDF) 4 Page - ELM Electronics |
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ELM322SM Datasheet(HTML) 4 Page - ELM Electronics |
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4 / 18 page  4 of 18 ELM322 ELM322DSE Elm Electronics – Circuits for the Hobbyist < http://www.elmelectronics.com/ > Overview The ELM322 relies on a standard RS232 type serial connection to communicate with the user. The data rate is fixed at 9600 baud, with 8 data bits, no parity bit, 1 stop bit, and no handshaking (often referred to as 9600 8N1). All responses from the IC are terminated with a single carriage return character and, by default, a line feed character as well. Make sure your software is configured properly for the mode you have chosen. Properly connected and powered, the ELM322 will initially display the message: ELM322 v2.0 > In addition to identifying the version of the IC, receipt of this string is a convenient way to be sure that the computer connections and the settings are correct. However, at this point no communications have taken place with the vehicle, so the state of the OBD connection is still unknown. The ‘>’ character displayed above is the ELM322’s prompt character. It indicates that the device is in its idle state, ready to receive characters on the RS232 port. Characters sent from the computer can either be intended for the ELM322’s internal use, or for reformatting and passing on to the vehicle’s OBD bus. Commands for the ELM322 are distinguished from those to the vehicle by always beginning with the characters ‘AT’ (as is common with modems), while commands for the OBD bus can contain only the ASCII characters for hexadecimal digits (0 to 9 and A to F). This allows the ELM322 to quickly determine where the received characters are to be directed. Whether an ‘AT’ type internal command or a hex string for the OBD bus, all messages to the ELM322 must be terminated with a carriage return character (hex ‘0D’) before they will be acted upon. If an incomplete string is sent and no carriage return appears, an internal timer will automatically abort the incomplete message after about 10 seconds. Should this happen, the ELM322 will print a single question mark to show that the input was not understood (and was ignored). Messages that are misunderstood by the ELM322 (syntax errors) will always be signalled by a single question mark (‘?’). These include incomplete messages, invalid hexadecimal digit strings, or incorrect AT commands. It is not an indication of whether or not the message was understood by the vehicle. (The ELM322 is a protocol interpreter that makes no attempt to assess OBD messages for validity – it only ensures that an even number of hex digits were received, combined into bytes, and sent out the OBD port, so it cannot determine if the message sent to the vehicle is in error.) Incomplete or misunderstood messages can also occur if the controlling computer attempts to write to the ELM322 before it is ready to accept the next command (as there are no handshaking signals to control the data flow). To avoid a data overrun, users should always wait for the prompt character (‘>’) before issuing the next command. Finally, a few convenience items to note. The ELM322 is not case-sensitive, so ‘ATZ’ is equivalent to ‘atz’, and to ‘AtZ’. The device ignores space characters as well as control characters (tab, linefeed, etc.) in the input, so they can be inserted anywhere to improve readability, and finally, issuing only a single carriage return character will repeat the last command (making it easier to request updates on dynamic data such as engine rpm). Communicating with the ELM322 The following describes how to use the ELM322 to obtain a great deal of information from your vehicle. To some, the quantity of information will be overwhelming, and for others it will not be enough. We begin by discussing just how to talk to the IC, then how to adjust some options through the use of ‘AT’ commands, and finally go on to actually talk to the vehicle, obtaining trouble codes and resetting them. For the more advanced experimenters, there are also sections on how to use some of the programmable features of this product as well. It is not as daunting as it first appears. Many users will never need to issue an ‘AT’ command, adjust timeouts or change the headers. For most, all that is required is a PC or a PDA with a terminal program (such as HyperTerminal or ZTerm), and knowledge of one or two OBD commands, which we provide in the following… |
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