612699

Elm 327 Manual

39
Zoom out
Zoom in
Previous page
1/94
Next page
Setting the Headers
The emissions related diagnostic trouble codes
that most people are familiar with are described in the
SAE J1979 standard (ISO15031-5). They represent
only a portion of the data that a vehicle may have
available – much more can be obtained if you are able
to direct the requests elsewhere.
Accessing most OBDII diagnostics information
requires that requests be made to what is known as a
a ‘functional address.’ Any processor that supports the
function will respond to the request (and theoretically,
many different processors can respond to a single
functional request). In addition, every processor (or
ECU) will also respond to what is known as their
physical address. It is the physical address that
uniquely identifies each module in a vehicle, and
permits you to direct more specific queries to only one
particular module.
To retrieve information beyond that of the OBDII
requirements then, it will be necessary to direct your
requests to either a different functional address, or to
an ECU’s physical address. This is done by changing
the data bytes in the message header.
As an example of functional addressing, let us
assume that you want to request that the processor
responsible for Engine Coolant provide the current
Fluid Temperature. You do not know its address, so
you consult the SAE J2178 standard and determine
that Engine Coolant is functional address 48. SAE
standard J2178 also tells you that for your J1850 VPW
vehicle, a priority byte of A8 is appropriate. Finally,
knowing that a scan tool is normally address F1, you
have enough information to specify the three header
bytes (A8 48 and F1). To tell the ELM327 to use these
new header bytes, all you need is the Set Header
command:
>AT SH A8 48 F1
OK
The three header bytes assigned in this manner
will stay in effect until changed by the next AT SH
command, a reset, or an AT D.
Having set the header bytes, you now need only
send the secondary ID for fluid temperature (10) at the
prompt. If the display of headers is turned off, the
conversation could look like this:
>10
10 2E
The first byte in the response echoes the request,
as usual, while the data that we requested is the 2E
byte. You may find that some requests, being of a low
priority, may not be answered immediately, possibly
39 of 94
ELM327
ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
encapsulated within a message, with ‘header’ bytes at
the beginning, and a ‘checksum’ at the end. The
J1850, ISO 9141-2, and ISO 14230-4 protocols all use
essentially the same structure, with three header
bytes, a maximum of seven data bytes and one
checksum byte.
The ISO 15765-4 (CAN) protocol uses a very
similar structure (see Figure 4), the main difference
really only relating to the structure of the header. CAN
header bytes are not referred to as header bytes –
they are called ‘ID bits’ instead. The initial CAN
standard defined the ID bits as being 11 in number,
while the more recent CAN standard now allows for
either 11 or 29 bit IDs.
The ELM327 does not normally show any of these
extra bytes unless you turn that feature on with the
Headers On command (AT H1). Issuing that allows
you to see the header bytes and the checksum byte
(for the J1850, ISO 9141 and ISO 14230 protocols).
For the CAN protocols, you will see the ID bits, and
other items which are normally hidden such as the PCI
byte for ISO 15765, or the data length codes (if they
are enabled with PP 29, or AT D1). Note that the
ELM327 does not display the checksum information
for CAN systems, or the IFR bytes for J1850 systems.
It is not necessary to ever have to set these
header byes, or to perform a checksum calculation, as
the ELM327 will always do this for you. The header
bytes are adjustable however, should you wish to
experiment with advanced messages such as those
for physical addressing. The next section provides a
discussion on how to do this…
OBD Message Formats (continued)
… 39 …


Need help? Post your question in this forum.

Forumrules


Report abuse

Libble takes abuse of its services very seriously. We're committed to dealing with such abuse according to the laws in your country of residence. When you submit a report, we'll investigate it and take the appropriate action. We'll get back to you only if we require additional details or have more information to share.

Product:

For example, Anti-Semitic content, racist content, or material that could result in a violent physical act.

For example, a credit card number, a personal identification number, or an unlisted home address. Note that email addresses and full names are not considered private information.

Forumrules

To achieve meaningful questions, we apply the following rules:

Register

Register getting emails for Elm 327 at:


You will receive an email to register for one or both of the options.


Get your user manual by e-mail

Enter your email address to receive the manual of Elm 327 in the language / languages: English as an attachment in your email.

The manual is 0,23 mb in size.

 

You will receive the manual in your email within minutes. If you have not received an email, then probably have entered the wrong email address or your mailbox is too full. In addition, it may be that your ISP may have a maximum size for emails to receive.

The manual is sent by email. Check your email

If you have not received an email with the manual within fifteen minutes, it may be that you have a entered a wrong email address or that your ISP has set a maximum size to receive email that is smaller than the size of the manual.

The email address you have provided is not correct.

Please check the email address and correct it.

Your question is posted on this page

Would you like to receive an email when new answers and questions are posted? Please enter your email address.



Info