ELM327
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:
Protocol
Description
Automatic
0
1
2
3
4
5
6
7
8
9
A
B
C
SAE J1850 PWM (41.6 kbaud)
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)
>AT SP 2
OK
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
User2 CAN (11* bit ID, 50* kbaud)
*user adjustable
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?
Figure 2. ELM327 Protocol Numbers
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:
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 SP A2
OK
>AT TP A2
OK
>AT DP
AUTO, SAE J1850 VPW
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).
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).
To have the ELM327 automatically search for a
protocol to use, simply send:
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.
>AT SP 0
OK
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
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
ELM327DSI
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