ELM329
AT Command Descriptions (continued)
FC SH wwxxyyzz
[ Flow Control Set Header to… ]
feature is most useful if you wish to perform the power
control functions using your own software. If you
disable the automatic response to a low input on this
pin (by setting bit 2 of PP 0E to 0), then pin 15 will
function as the RTS input. A low level on the input will
not turn the power off, but it will interrupt any OBD
activity that is in progress. All you need to do is detect
the ‘STOPPED’ message that is sent when the
ELM329 is interrupted, and then check the level at pin
15 using AT IGN. If it is found to be OFF, you can
perform an orderly shutdown yourself.
This command is used to set the header (or ‘CAN
ID’) bits for Flow Control responses with 29 bit CAN ID
systems. Since the 8 nibbles define 32 bits, only the
right-most 29 bits of those provided will be used - the
most significant three bits are always ignored. This
command only affects Flow Control mode 1.
FC SM h
[ Flow Control Set Mode to h ]
This command sets how the ELM329 responds to
First Frame messages when automatic Flow Control
responses are enabled. The single digit provided can
either be ‘0’ (the default) for fully automatic responses,
‘1’ for completely user defined responses, or ‘2’ for
user defined data bytes in the response. Note that FC
modes 1 and 2 can only be enabled if you have
defined the needed data and possibly ID bytes. If you
have not, you will get an error message. More
complete details and examples can be found in the
Altering Flow Control Messages section (page 54).
IN1
[ read the level at INput 1 ]
This command causes the ELM329 to read the
logic level at pin 12. If it is at a low level, ‘0’ will be
reported, while a high level results in a ‘1’. The level
shown is subject to the hysteresis effects of the
Schmitt trigger wave shaping of the input circuitry.
IN2
[ read the level at INput 2 ]
This command causes the ELM329 to read the
H0 and H1
[ Headers off or on ]
logic level at pin 13. If it is at a low level, ‘0’ will be
reported, while a high level results in a ‘1’. The level
shown is subject to the hysteresis effects of the
Schmitt trigger wave shaping of the input circuitry.
These commands control whether or not the
header (ID and possibly DLC) bytes of information are
shown in the responses from the vehicle. These are
not normally shown by the ELM329, but may be of
interest (especially if you receive multiple responses
and wish to determine what modules they were from).
Turning the headers on (with AT H1) actually
shows more than just the header bytes – you will see
the complete message as transmitted, including the
PCI bytes, and the CAN data length code (DLC) if it
has been enabled. The current version of this IC does
not display the CAN CRC code.
JE
[ enables the J1939 ELM data format ]
The J1939 standard requires that PGN requests
be sent with the byte order reversed from the standard
‘left-to-right’ order, which many of us would expect. For
example, to send a request for the engine temperature
(PGN 00FEEE), the data bytes are actually sent in the
reverse order (ie EE FE 00), and the ELM329 would
normally expect you to provide the data in that order
for passing on to the vehicle.
I
[ Identify yourself ]
When experimenting, this constant need for byte
reversals can be quite confusing, so we have defined
an ELM format that reverses the bytes for you. When
the J1939 ELM (JE) format is enabled, and you have a
J1939 protocol selected, and you provide three data
bytes to the ELM329, it will reverse the order for you
before sending them to the ECU. To request the
engine temperature PGN, you would send 00 FE EE
(and not EE FE 00). The ‘JE’ type of automatic
formatting is enabled by default.
Issuing this command causes the chip to identify
itself, by printing the startup product ID string (currently
‘ELM329 v2.0’). Software can use this to determine
exactly which integrated circuit it is talking to, without
having to reset the IC.
IGN
[ read the IgnMon input level ]
This command reads the signal level at pin 15. It
assumes that the logic level is related to the ignition
voltage, so if the input is at a high level, the response
will be ‘ON’, and a low level will report ‘OFF’. This
JHF0 and JHF1 [ J1939 Header Formatting off or on ]
When printing responses, the ELM329 normally
ELM329DSB
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