ELM327
Using Higher RS232 Baud Rates (continued)
The BRD command allows a new baud rate divisor
to be tested, and then accepted or rejected depending
on the results of the test. See the chart at the right,
which shows how the command works.
PC
ELM327
As can be seen, the software first makes a request
for a new baud rate divisor, using this AT command.
For example, to try the 57.6K rate that was previously
discussed, the controlling PC would send:
Request for a new
baud rate divisor:
AT BRD hh
ELM327 responds
with ‘OK’
(if it is supported)
AT BRD 45
If the ELM327 is an older version, it will not
support this command, and will return with the familiar
‘?’ character. If it does support the command, it will
respond with ‘OK’, so the software knows whether to
proceed or not. No prompt character follows the ‘OK’
reply; it is followed only by a carriage return character
(and optionally, a linefeed character).
Program switches to
the new baud rate,
and waits for input
ELM327 switches to
new baud rate and
waits for 75 msec*
Having sent an ‘OK’, the ELM327 then switches to
the new (proposed) baud rate, and then simply waits a
predetermined time (nominally 75 msec). This period
is to allow the PC sufficient time to change its baud
rate. When the time is up, the ELM327 then sends the
ID string (currently ‘ELM327 v2.0’) to the PC at the
new baud rate (followed by a carriage return) and
waits for a response.
ELM327 sends
the AT I string
If the Rx is good,
Program sends a
carriage return
ELM327 waits
up to 75 msec*
Knowing that it should receive the ELM327 ID
string, the PC software compares what was actually
received to what was expected. If they match, the PC
responds with a carriage return character, but if there
is a problem, the PC sends nothing. The ELM327 is
meanwhile waiting for a valid carriage return character
to arrive. If it does (within 75 msec), the proposed
baud rate is retained, and the ELM327 says ‘OK’ at
this new rate. If it does not see the carriage return, the
baud rate reverts back to the old rate. Note that the PC
might correctly output the carriage return at this new
rate, but the interface circuitry could corrupt the
character, and the ELM327 might not see a valid
response, so your software must check for an ‘OK’
response before assuming that the new rate has been
accepted.
for a carriage return
CR
received
?
yes
no
Baud rate reverts
to the previous
baud rate
ELM327 says ‘OK’
(and remains at the
new baud setting)
Print a prompt,
and wait for the
next command
Using this method, a program can quickly try
several baud rates, and determine the most suitable
one for the connected hardware. The new baud rate
will stay in effect until reset by an AT Z, a Power
Off/On, or a MCLR input. It is not affected by the AT D
(set Defaults), or AT WS (Warm Start) commands.
* the 75 msec time is adjustable
with the AT BRT hh command
ELM327DSI
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