ELM327
CAN Message Formats
The ISO 15765-4 (CAN) standard defines several
message types that are to be used with diagnostic
systems. Currently, there are four that may be used:
First Frame message. The length (014) was actually
extracted from that message by the ELM327 and
printed on the first line as shown. Following the First
Frame line are two Consecutive Frames (that begin
with 1: and 2:). To learn more details of the exact
formatting, you may want to send a request such as
the one above, then repeat the same request with the
headers enabled (AT H1). This will show the PCI bytes
that are actually used to send these components of the
total message.
The Flow Control frame is one that you do not
normally have to deal with. When a First Frame
message is sent as part of a reply, the ELM327 must
tell the sender some technical things (such as how
long to delay between Consecutive Frames, etc.) and
does so by replying immediately with a Flow Control
message. These are predefined by the ISO 15765-4
standard, so can be automatically inserted for you. If
you wish to generate custom Flow Control messages,
then refer to the ‘Altering Flow Control Messages’
section, on page 48.
If a Flow Control frame is detected while
monitoring, the line will be displayed with ‘FC: ’ before
the data, to help you with decoding of the information.
There is a final type of message that is
occasionally reported, but is not supported by the
diagnostics standard. The (Bosch) CAN standard
allows for the transmission of a data request without
sending any data in the requesting message. To
ensure that the message is seen as such, the sender
also sets a special flag in the message (the RTR bit),
which is seen at each receiver. The ELM327 always
looks for this flag, or for zero data bytes, and may
report to you that an RTR was detected while
monitoring. This is shown by the characters RTR
where data would normally appear, but only if the CAN
Auto Formatting is off, or headers are enabled. Often,
when monitoring a CAN system with an incorrect baud
rate chosen, RTRs may be seen.
Note that the CAN system is quite robust with
several error detecting methods in place, so that
during normal data transmission you will rarely see
any errors. When monitoring buses however, you may
well see errors (especially if the ELM327 is set to an
incorrect baud rate). As a diagnostic aid, when errors
do occur, the ELM327 will print all bytes (no matter
what CAF, etc., is set to), followed by the message
‘<RX ERROR’.
SF - the Single Frame
FF - the First Frame (of a multiframe message)
CF - the Consecutive Frame ( ‘ ‘ )
FC - the Flow Control frame
The Single Frame message contains storage for
up to seven data bytes in addition to what is known as
a PCI (Protocol Control Information) byte. The PCI
byte is always the first of the data bytes, and tells how
many data bytes are to follow. If the CAN Auto
Formatting option is on (CAF1) then the ELM327 will
create this byte for you when sending, and remove it
for you when receiving. (If the headers are enabled,
you will see it in the responses.)
If you turn the Auto Formatting off (with CAF0), it
is expected that you will provide all of the data bytes to
be sent. For diagnostics systems, this means the PCI
byte and the data bytes. The ELM327 will not modify
your data in any way, except to add extra padding
bytes for you, to ensure that you always send as many
data data bytes as are required (eight for ISO15765).
You do not need to set the Allow Long (AT AL) option
in order to send eight bytes, as the IC overrides it for
you.
A First Frame message is used to say that a multi-
frame message is about to be sent, and tells the
receiver just how many data bytes to expect. The
length descriptor is limited to 12 bits, so a maximum of
4095 byes can be received at once using this method.
Consecutive Frame messages are sent after the
First Frame message to provide the remainder of the
data. Each Consecutive Frame message includes a
single hex digit ‘sequence number’ that is used to
determine the order when reassembling the data. It is
expected that if a message were corrupted and resent,
it could be out of order by a few packets, but not by
more than 16, so the single digit is normally more than
adequate. As seen previously, the serial number for a
vehicle is often a multiframe response:
>0902
014
0: 49 02 01 31 44 34
1: 47 50 30 30 52 35 35
2: 42 31 32 33 34 35 36
In this example, the line that begins with 0: is the
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