Manual Elm 327 (page 36 of 94) (English)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

Selecting Protocols

The ELM327 supports several different OBD

protocols (see Figure 2, at right). As a user, you may

never have to choose which one it should use (since

the factory settings cause an automatic search to be

performed for you), but while experimenting, you may

want to specify a protocol to be used.

For example, if you know that your vehicle uses

the SAE J1850 VPW protocol, you may want the

ELM327 to use only that protocol, and no others. If

that is what you want, simply determine the protocol

number (from Figure 2), then use the ‘Set Protocol’ AT

Command as follows:

>AT SP 2

From this point on, the default protocol (used after

every power-up or AT D command) will be protocol 2

(or whichever one that you have chosen). Verify this

by asking the ELM327 to describe the protocol:

>AT DP

SAE J1850 VPW

Now what happens if your friend has a vehicle that

uses ISO 9141-2? How do you now use the ELM327

interface for that vehicle, if it is set for J1850?

One possibility is to change your protocol selection

to allow for the automatic searching for another

protocol, on failure of the current one. This is done by

putting an ‘A’ before the protocol number, as follows:

>AT SP A2

>AT DP

AUTO, SAE J1850 VPW

Now, the ELM327 will try protocol 2, but will then

automatically begin searching for another protocol

should the attempt to connect with protocol 2 fail (as

would happen when you try to connect to your friend’s

vehicle).

The Set Protocol commands cause an immediate

write to the internal EEPROM, before even attempting

to connect to the vehicle. This write is time-consuming,

affects the setting for the next powerup, and may not

actually be appropriate, if the protocol selected is not

correct for the vehicle. To allow a test before a write

occurs, the ELM327 offers one other command - the

Try Protocol (TP) command.

Try Protocol is very similar to Set Protocol. It is

used in exactly the same way as the AT SP command,

the only difference being that a write to internal

memory will only occur after a valid protocol is found,

and only if the memory function is enabled (M0/M1).

For the previous example, all that needs to be sent is:

>AT TP A2

Many times, it is very difficult to even guess at a

protocol to try first. In these cases, it is best to simply

let the ELM327 decide what to use. This is done by

telling it to use protocol 0 (with either the SP or the TP

commands).

To have the ELM327 automatically search for a

protocol to use, simply send:

>AT SP 0

and when the next OBD command is to be sent, the

ELM327 will automatically look for one that responds.

You will see a ‘SEARCHING...’ message, followed by

a response, after which you can ask the ELM327 what

protocol it found (by sending AT DP).

The first versions of the ELM327 used the SAE

recommended search order (protocol 1, 2, 3, etc.), but

recent versions of the IC modify the search order

Figure 2. ELM327 Protocol Numbers

Description

SAE J1850 PWM (41.6 kbaud)

Protocol

Automatic

SAE J1850 VPW (10.4 kbaud)

ISO 9141-2 (5 baud init)

ISO 14230-4 KWP (5 baud init)

ISO 14230-4 KWP (fast init)

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

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

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

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

SAE J1939 CAN (29 bit ID, 250* kbaud)

User1 CAN (11* bit ID, 125* kbaud)

User2 CAN (11* bit ID, 50* kbaud)

*user adjustable

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Elm 327 Manual

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