ELM325
AT Command Descriptions
The following describes each AT Command that the
current version of the ELM325 supports, in some
detail. Many of these commands are also described
further in other sections:
<CR>
[ repeat the last command ]
checksum byte visible or not. Usually, it is preferred
that the byte not be shown, so the ELM325 hides it by
default. If you wish to see the byte printed with every
message, simply send AT C1.
Sending a single carriage return character causes
the ELM325 to repeat the last command that it
performed. This helps if you wish to repeat a request
several times to obtain updates for a monitored value,
for example.
D
[ set all to Defaults ]
This command is used to set the options to their
AF0 and AF1
[ Auto Formatting off or on* ]
default (or factory) settings, as when power is first
applied. Any settings that you had made for a custom
MID, for filters, for message formatting, or timer
settings will be restored to their default values.
All messages must start with the sender’s address
(the MID) and end with a checksum. If automatic
formatting is on (it is by default), the ELM325 will add
these to every message for you, so that you only need
to provide the data bytes that you wish to send. If
automatic formatting is off, you must provide all bytes,
including the MID and the checksum.
Automatic (ie ‘auto’) formatting also provides help
when making requests for a PID. If you provide one
data byte, or two data bytes with the first being 01, the
ELM325 assumes that you wish to make a PID
request. It will add the MID, the request PID,
checksum, etc. for you, set the filters if you haven’t,
and send the request. If the auto formatting is off, all
bytes that you provide are sent without modification of
any kind (but the ELM325 still attempts to set filters for
you, if you have not set any).
One final issue to note is that the ELM325’s
internal J1708 buffer is 21 bytes long. This means that
you may actually send a 23 byte message if formatting
is on (MID + 21 bytes in buffer + checksum), but if the
auto formatting is off, the total length of the message
must be 21 bytes or less (as the entire message is
taken from the buffer). It also means that if you try to
provide 21 data bytes for a message, plus a message
count nibble, you will get an error (as you have over-
filled the internal buffer). This is generally of no
concern if you are implementing the SAE J1587
protocol, but may be an issue if you are trying to send
some special SAE J1922 messages.
E0 and E1
[ Echo off or on* ]
These commands control whether or not the
characters received on the RS232 port are echoed
(retransmitted) back to the host computer. Character
echo can be used to confirm that the characters sent
to the ELM325 were received correctly. The default is
E1 (or echo on).
EM0 and EM1
[ Error Messages off or on* ]
If a message is received and there is a problem
with it, the ELM325 will add an error description to the
received line. An arrow (‘<‘) that points to the message
is printed first, then either PROT ERROR, RX ERROR,
or DATA ERROR is printed to describe the problem.
You may find that the description is of little use to you
in your application, and wish to only see the ‘arrow’. If
you set AT EM0, the ELM325 will do that for you.
F1
[ reset Filter 1 ]
If you have set message receive filter F1 and wish
to disable that setting, simply send AT F1. The
ELM325 will discard any setting that you had made.
F2
[ reset Filter 2 ]
If you have set message receive filter F2 and wish
to disable that setting, simply send AT F2. The
ELM325 will discard any setting that you had made.
C0 and C1
[ Checksum display off* or on ]
These commands are used to make the received
ELM325DSA
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