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ELM320SM Datasheet(PDF) 9 Page - ELM Electronics |
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ELM320SM Datasheet(HTML) 9 Page - ELM Electronics |
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9 / 16 page  9 of 16 ELM320DSD Elm Electronics – Circuits for the Hobbyist < http://www.elmelectronics.com/ > Talking to the Vehicle The ELM320 cannot be directly connected to a vehicle as it is, but needs support circuitry as shown in the Example Applications section. Once incorporated into such a circuit, you only need to use a terminal program to send bytes to, and receive them from, the vehicle. SAE standards specify that command bytes sent to the vehicle must adhere to a set format. The first byte (known as the ‘mode’) always describes the type of data being requested, while the second, third, etc. bytes specify the actual information required (given by a ‘parameter identification’ or PID number). The modes and PIDs are described in detail in the SAE standard documents J1979 and J2190, and may also be expanded on by the vehicle manufacturers. Normally, one is only concerned with the nine diagnostic test modes described in J1979 (although there is provision for more). Note that it is not a requirement for all of them to be supported. These are the nine modes: 01 : show current data 02 : show freeze frame data 03 : show diagnostic trouble codes 04 : clear trouble codes and stored values 05 : test results, oxygen sensors 06 : test results, non-continuously monitored 07 : test results, continuously monitored 08 : special control mode 09 : request vehicle information Within each mode, PID 00 is normally reserved to show which PIDs are supported by that mode. Mode 01, PID 00 is required to be supported by all vehicles, and can be accessed as follows… Ensure that the ELM320 is properly connected to your vehicle, and powered. Most vehicles will not respond without the ignition key in the ON position, so turn the ignition on, but do not start the vehicle. At the prompt, issue the mode 01 PID 00 command: >01 00 A typical response could be as follows: 41 00 BE 1F B8 10 The 41 00 signifies a response (4) from a mode 1 request with PID 00 (a mode 2, PID 00 request is answered with a 42 00, etc.). The next four bytes (BE, 1F, B8, and 10) represent the requested data, in this case a bit pattern showing which of PIDs 1 through 32 are supported by this mode (1=supported, 0=not). Although this information is not very useful for the casual user, it does serve to show that you are communicating with the vehicle. Another example requests the current engine coolant temperature (ECT). This is PID 05 in mode 01, and is requested as follows: >01 05 The response will be of the form: 41 05 7B This shows a mode 1 response (41) from PID 05, with value 7B. Converting the hexidecimal 7B to decimal, one gets 7 x 16 + 11 = 123. This represents the current temperature in degrees Celsius, with the zero value offset by 40 degrees to allow operation at subzero temperatures. To convert to the actual coolant temperature, simply subtract 40 from the value. In this case, then, the ECT is 123 - 40 = 83 degrees Celsius. A final example shows a request for the OBD requirements to which this vehicle was designed. This is PID 1C of mode 01, so at the prompt, type: >01 1C A typical response would be: 41 1C 01 The returned value (01) shows that this vehicle conforms to OBDII (California ARB) standards. The presently defined responses are : 01 : OBDII (California ARB) 02 : OBD (Federal EPA) 03 : OBD and OBDII 04 : OBD I 05 : not intended to meet any OBD requirements 06 : EOBD (Europe) Some modes may provide multi-line responses (09, if supported, can display the vehicle’s serial number). The ELM320 will attempt to display all responses in these cases, but only if it is allowed sufficient time to process each. There may be occasions when the vehicle responds too quickly to allow time for reprocessing, and lines could be lost. Hopefully this has shown how typical requests proceed. It has not been meant to be a definitive source on modes and PIDs – this information can be obtained from the manufacturer of your vehicle, from the SAE (www.sae.org), ISO (www.iso.org), or from various other sources on the web. ELM320 |
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