ELM327P datasheet(15/51 Pages) ELM

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AT Commands (continued)

SH xyz

[ Set the Header to 00 0x yz ]

Entering CAN 11 bit ID words (headers) normally

requires that extra leading zeros be added (eg. AT SH

00 07 DF), but this command simplifies doing so. The

AT SH xyz command accepts a three digit argument,

takes only the right-most 11 bits from that, adds

leading zeros, and stores the result in the header

storage locations for you. As an example, AT SH 7DF

is a valid command, and is quite useful for working

with 11 bit CAN systems. It actually results in the

header bytes being internally stored as 00 07 DF.

SP h

[ Set Protocol to h ]

This command is used to set the ELM327 for

operation using the protocol specified by 'h', and to

also save it as the new default. Note that the protocol

will be saved no matter what the AT M0/M1 setting is.

The currently valid protocols are:

0 - Automatic

1 - SAE J1850 PWM (41.6 Kbaud)

2 - SAE J1850 VPW (10.4 Kbaud)

3 - ISO 9141-2 (5 baud init, 10.4 Kbaud)

4 - ISO 14230-4 KWP (5 baud init, 10.4 Kbaud)

5 - ISO 14230-4 KWP (fast init, 10.4 Kbaud)

6 - ISO 15765-4 CAN (11 bit ID, 500 Kbaud)

7 - ISO 15765-4 CAN (29 bit ID, 500 Kbaud)

8 - ISO 15765-4 CAN (11 bit ID, 250 Kbaud)

9 - ISO 15765-4 CAN (29 bit ID, 250 Kbaud)

A - SAE J1939 CAN (29 bit ID, 250* Kbaud)

B - USER1 CAN (11* bit ID, 125* Kbaud)

C - USER2 CAN (11* bit ID, 50* Kbaud)

* default settings (user adjustable)

The first protocol shown (0) is a convenient way of

telling the ELM327 to automatically try all protocols,

when looking for a valid one. It causes the ELM327 to

sequence through each of the protocols, looking for

one that can be initiated correctly. When a valid

protocol is found, and the memory function is enabled,

that protocol will then be remembered, and will

become the new default setting. When saved like this,

the automatic mode searching will still be enabled, and

the next time the ELM327 fails to connect to the saved

protocol, it will again search all protocols for another

valid one.

If another protocol (other than the Automatic one)

is selected with this command (eg. AT SP 3), that

protocol will become the default, and will be the only

protocol used by the ELM327. Failure to initiate a

connection in this situation will result in familiar

responses such as ‘BUS INIT: ...ERROR’, and no

other protocols will be attempted. This is a useful

setting if you know that your vehicle(s) only require the

one protocol.

SP Ah

[ Set Protocol to Auto, h ]

This variation of the SP command allows you to

choose a starting (default) protocol, while still retaining

the ability to automatically search for a valid protocol

on a failure to connect. For example, if your vehicle is

ISO 9141-2, but you want to occasionally use the

ELM327 circuit on other vehicles, you might set

AT SP A3. The default protocol will then be 3, but with

the ability to automatically search for other protocols.

Don't forget to disable the memory function if doing

this, or your neighbour’s protocol could become your

new default. As for AT SP h, an AT SP Ah will save

the protocol information even if the memory option is

off. Note that the ‘A’ can come before or after the h, so

AT SP A3 can also be entered as AT SP 3A.

SR hh

[Set the Receive address to hh ]

Depending on the application, users may wish to

manually set the address to which the ELM327 will

respond. Issuing this command will turn off the AR

mode, and force the IC to only accept responses

addressed to hh. Use caution with this setting, as

depending on what you set it to, you may end up

accepting (acknowledging with an IFR) a message that

was actually meant for another module.

This command does not affect addresses used by

the J1939 protocol. The J1939 routines derive the

required addresses from the header values, as

required by the SAE standard.

ST hh

[ Set Timeout to hh ]

After sending a request, the ELM327 waits a

preset time for a response before it can declare that

there was ‘NO DATA’ from the vehicle. The same

timer setting is also used after a response has been

received, to be sure that no more responses are

coming. The AT ST command allows this time to be

adjusted, in increments of about 4 msec.

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