ELM329LP datasheet(33/87 Pages) ELM

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ELM329L

ELM329L DSA

Elm Electronics – Circuits for the Hobbyist

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Selecting Protocols

The ELM329 supports several different OBD

protocols (see Figure 2, at right). This is a little

misleading however, as the ELM329 only provides

very minimal support for protocols 1 to 5 - they are

only included so that most ELM327 software will still

work with the ELM329.

The ELM329 really only provides support for CAN

protocols 6 to F. You may never need to actually

select one of these, since the factory settings cause

an automatic search to be performed for you, and the

protocol is activated if it seems appropriate. If

experimenting, you may wish to be able to select a

protocol, however.

For example, if you know that your vehicle uses

the CAN (ISO 15765-4) standard, with an 11 bit ID and

a rate of 500kbps (i.e. protocol 6), then you may want

the ELM329 to use only that one, and no others. If that

is what you want, simply use the ‘Set Protocol’ AT

Command as follows:

>AT SP 6

From that point on, the default protocol (used after

every power-up or AT D command) will be protocol 6

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

by asking the ELM329 to describe the protocol:

>AT DP

ISO 15765-4 (CAN 11/500)

Now what happens if your friend has a vehicle that

uses a different baud rate? How do you now use the

ELM329 interface for that vehicle, if it is set for your

car?

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 A6

>AT DP

AUTO, ISO 15765-4 (CAN 11/500)

Now, the ELM329 will try protocol 6, but will then

automatically begin searching for another protocol

should the attempt to connect with protocol 6 fail (as

might 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 ELM329 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 (see AT

M0/M1). For the previous example, all that needs to be

sent is:

>AT TP A6

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 ELM329 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 ELM329 automatically search for a

Figure 2. ELM329 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

SAE J1939* CAN (29* bit ID, 500* kbaud)

User4 CAN (11* bit ID, 95.2* kbaud)

User5 CAN (11* bit ID, 33.3* kbaud)

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