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ELM325P Datasheet(PDF) 11 Page - ELM Electronics |
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ELM325P Datasheet(HTML) 11 Page - ELM Electronics |
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11 / 31 page  Sending AT Commands Before learning some J1587 Commands, we will show a few examples of how to use an AT Command. We will assume that you have built (or purchased) a circuit that is similar to that of Figure 7 in the Example Applications section. Do not connect your circuit to a vehicle at this time. Power it from a test source only (a 9V ‘transistor radio’ battery works well), and connect it to your PC as discussed in the Communicating with the ELM325 section. For your first command, simply reset the IC by sending AT Z. Try this a few ways (don’t forget to press enter or return after each): >AT Z or >atz or >a T z You should see the RS232 LEDs blink as you type each letter, and after you press return (or enter) you should see all four Tx/Rx LEDs blink, in order, followed by a final blink of the RS232 Tx LED as the ELM325 sends ‘ELM325 v1.0’. Most J1708 messages are continually sent on the data bus, at a predetermined rate. Some messages may be sent 10x per second, while others are only 1x per 10 seconds. This means that you may need to adjust the internal timeout setting depending on what message you are attempting to receive. We will adjust this timeout setting next. Try this request for trouble codes: >AT TC After about 2 seconds, you should see a response that looks like: NO DATA > since there is no vehicle attached, there was no data received. Now, adjust the timeout to 10 seconds. If you look at the AT command list, you will see that there is a Set Timeout command that is used for this. Timing is in increments of 100 msec (0.1 sec), so to obtain a 10 second delay, the AT ST setting should be 100. We need to convert the 100 to hexadecimal, however, as all numbers handled by the ELM325 must be in hexadecimal. Converting then, 100 (= 6 x 16 + 4) is 64 in hexadecimal, so we send: >AT ST 64 Again, don’t forget to press return (or enter). Now, the timeout should be set to 10 seconds. To verify this, repeat the Trouble Codes command: >AT TC It should be 10 seconds before you see the NO DATA response this time. To try it again, you do not need to enter the AT TC command again, you only need to press enter, and the ELM325 will repeat your last command (AT TC) for you. As a final test, enter AT TC again, but before the 10 seconds is up, press any key on the keyboard. You should see the ELM325 respond with: STOP? which means that it was interrupted and it thinks that you wish to stop. If you ever see the ‘STOP?’ response, it means that the ELM325 thinks it has been interrupted by you. Now, restore the AT ST time to the default value, with the Defaults command: >AT D You should see a response of ‘OK’ and then a prompt character on a new line, to show that the ELM325 is waiting for you. In this case, you might also have sent AT ST 00, since that also restores the timeout setting to its default value. Experiment with these commands – you are not connected to a vehicle, so can do no harm. Sending AT Commands is not difficult, they just require a little practice. 11 of 31 ELM325 ELM325DSA Elm Electronics – Circuits for the Hobbyist www.elmelectronics.com |
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