Manual Elm 327 (page 26 of 94) (English)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

AT Command Descriptions (continued)

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 that the vehicle’s protocol is not

known, and that it should perform a search. It causes

the ELM327 to try all protocols if necessary, 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. Note that some vehicles respond to more

than one protocol - if searching, you may see more

than one type of response.

ELM327 users often use the AT SP 0 command to

reset the search protocol before starting (or restarting)

a connection. This works well, but since it is used so

often, and since writes to EEPROM result in an

unnecessary delay (of about 30 msec), the AT SP0

command sets the protocol to 0, but does not perform

a write to EEPROM. Similarly, the SP A0 and SP 0A

commands do not perform writes to EEPROM, either.

Saving this value to EEPROM would not provide any

advantage (and would be very short-lived, as the

ELM327 will soon be finding the vehicle’s protocol and

over-writing the ‘0’ value in EEPROM). If you really

want to store the value ‘0’ in the internal EEPROM,

you must use the AT SP 00 command.

If another protocol (other than 0) 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 a response 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 use the one protocol, but is also one that can

cause a lot of problems if you do not understand it.

SP 00 [ erase the Stored Protocol ]

To speed up protocol initiation and detection, the

SP 0 command sets the protocol to automatic, but

does not perform a (very time-consuming) write to

EEPROM. Some users felt it was necessary to be able

to actually write to the ELM327's EEPROM, however,

so we provided this command. It should not normally

be used when connecting to a vehicle.

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 use the

AT SP A3 command, so that the first protocol tried will

then be yours (3), but it will also automatically search

for other protocols. Don't forget to disable the memory

function if doing this, or each new protocol detected

will become your new default.

SP Ah will save the protocol information even if

the memory option is off (but SP A0 and SP 0A do not-

if you must write 0 to the EEPROM, use command AT

SP 00). 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 accept a

message that was actually meant for another module,

possibly sending an IFR when you should not. To turn

off the SR filtering, simply send AT AR.

This command has limited use with CAN, as it only

monitors one byte of the ID bits, and that is not likely

selective enough for most CAN applications (the CRA

command may be a better choice). Also, the command

has no effect on the addresses used by the J1939

protocols, as the J1939 routines set their own receive

addresses based on the ID bit (header) values.

This SR command is exactly the same as the RA

command, and can be used interchangeably with it.

Note that CAN Extended Addressing does not use this

value - it uses the one set by the AT TA command.

SS [ use the Standard Sequence for searches ]

SAE standard J1978 specifies a protocol search

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