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ELM322P Datasheet(PDF) 8 Page - ELM Electronics |
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ELM322P Datasheet(HTML) 8 Page - ELM Electronics |
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8 / 18 page  If the bytes received on the RS232 bus do not begin with the letters A and T, they are assumed to be commands for the vehicle’s OBD bus. The bytes will be tested to ensure that they are valid pairs of hexadecimal digits and, if they are, will be combined into bytes for transmitting to the vehicle. Recall that no checks are made as to the validity of the OBD command – data is simply retransmitted as received. OBD commands are actually sent to the vehicle embedded within a data message. The J1979 standard requires that every message begin with three header bytes followed by the data bytes, and finally be terminated with a checksum byte, but the ELM322 takes care of this formatting for you. It powers on expecting to be used for OBDII mandated emissions diagnostics, so knows the values necessary for the header bytes, sets them accordingly, and simply has to insert the user’s data. If you wish to experiment with some of the more advanced functions, the values used for the header bytes may be changed with AT commands, as discussed previously. To view these extra bytes as they are received, you must turn the header display on by issuing an ATH1 command. The command portion of most OBD messages is usually only one or two bytes in length, but can occasionally be longer, as the standard allows for as many as seven. The current version of the ELM322 will accept the maximum seven command bytes (or 14 hexadecimal digits) per message, while users of previous versions (v1.x) were limited to only three command bytes. In either case, attempts to send more than the maximum number of bytes allowed will result in a syntax error, with the entire command being ignored and a single question mark printed. The use of hexadecimal digits for all of the data exchange was chosen as it is the most common data format used in the relevant SAE standards. It is consistent with mode request listings and is the most frequently used format for displaying results. With a little practice, it should not be very difficult to deal in hex numbers, but some may initially find the table in Figure 2 or a calculator to be invaluable. All users will eventually be required to manipulate the results in some way, though (combine bytes and divide by 4 to obtain rpm, divide by 2 to obtain degrees of advance, etc.), and may find a software front-end helpful. As an example of sending a command to the vehicle, assume that A6 (or decimal 166) is the command that is required to be sent. In this case, the user would type the letter A, then the number 6, then would press the return key. These three characters would be sent to the ELM322 on the RS232 bus. The ELM322 would store the characters as they are received, and when the third character (the carriage return) is received, begin to assess the other two. It would see that they are both valid hex digits, and would convert them to a one byte value (with a decimal value of 166). Three header bytes and a checksum byte would be added, so a total of five bytes would be sent to the vehicle. Note that the carriage return character is only a signal to the ELM322, and is not sent on to the vehicle. After sending a command, the ELM322 listens to the OBD bus for any responses that are directed to it. Each received byte is converted to the equivalent hexadecimal pair of ASCII characters and transmitted on the RS232 port for the user. Rather than send control characters which are unprintable on most terminals, the digits are sent as numbers and letters (e.g. the hex digit ‘A’ is transmitted as decimal value 65, and not 10). If there was no response from the vehicle, due to no data being available, or because the command is not supported, a ‘NO DATA’ message will be sent. See the Error Messages section for a description of this message and others. 8 of 18 ELM322 ELM322DSE Elm Electronics – Circuits for the Hobbyist < http://www.elmelectronics.com/ > OBD Commands Figure 2. Hex to Decimal Conversion Hexadecimal Number Decimal Equivalent 0 1 3 2 4 5 6 0 1 3 2 4 5 6 7 7 8 8 9 9 A 10 B 11 C 12 D 13 E 14 F 15 |
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