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ELM634 Datasheet(PDF) 7 Page - ELM Electronics |
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ELM634 Datasheet(HTML) 7 Page - ELM Electronics |
7 / 11 page 7 of 11 ELM634 ELM634DSB Elm Electronics – Circuits for the Hobbyist < http://www.elmelectronics.com/ > Monitoring the LIN Bus Data bytes that appear on the LIN bus can assume values from 0 to 255. These cannot be displayed using a PC terminal program, however, since many of these values are not printable characters. In order to make them readable, the ELM634 re-formats every byte as a pair of hexadecimal digits, using standard ASCII characters. A typical request made of the ELM634 would appear as: >AT MA 49: 6C 8F 04 The AT MA is the user’s request to ‘monitor all’, while the 49: 6C 8F 04 is what the ELM634 found on the LIN bus. Note that the initial Synch Byte is always received, but is never displayed (it is always 55). The identifier byte (49 in this example) always appears first, and is separated from the data bytes by a colon character (“:”), while the pairs of hexadecimal digits following represent the data bytes that were received. The final pair of digits on each line is the checksum byte (04 in this case). Should the checksum byte not match the value calculated internally by the ELM634, the error will be flagged by printing a single question mark at the end of the line. For an example, if the slave driver in the above case was weak, allowing an extra ‘1’ to appear in the first data byte of this example, the output might typically look like: >AT MA 49: 6C 8F 04 49: 7C 8F 04? The question mark at the end of the second response line alerts you to the fact that an error is present, but the position of the error cannot be determined from this information – you will only be able to say that it is somewhere in the response. Another type of error that could occur is when a slave fails to respond to the master. In this case, you would typically see only the identifier, followed by a single question mark: >AT MA 49: The question mark gets printed in this case because 49 is not a valid checksum byte (all bytes must add up to FF). Often, this would be followed by a timeout so the output could appear as: >AT MA 49: [T] As you experiment, you will likely find that timeouts are a very common occurence. They simply mean that there has been no activity for some time, often due to the system going into a low-power sleep mode. Error messages are described further in the next section. This data is typical of what might be experienced in many systems. The current version of the LIN standard (2.0) allows for the possibility of an arbitrary number of data bytes however, so while the ELM634 has no limitations on what it can display, the user should be prepared for this possibility. This could involve simply allocating enough buffer space, processing the data faster than it arrives, or by simply ignoring lines that are longer than a predetermined length. The ELM634 also supports one other type of monitoring command, which is useful when you know the specific responses that you wish to view. It is the AT MR command, which requests that only specific responses to ID bytes be displayed, while ignoring all others. For instance, a ‘monitor all’ command might have resulted in: >AT MA D3: C0 00 3F 49: 6C 8F 04 92: D4 00 2B D3: C0 00 3F . . . Only a portion of the data stream is shown, as it can typically be quite lengthy. However, if the user was only interested in responses that began with 92 for instance, they need only issue an AT MR 92 command to filter the information for them. The above would have looked like: >AT MR 92 92: D4 00 2B This command is often helpful when trying to diagnose a particular problem. |
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