ELM327P datasheet(60/76 Pages) ELM

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ELM327

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ELM327DSH

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Using Higher RS232 Baud Rates (continued)

* the 75 msec time is adjustable

with the AT BRT hh command

we have provided the BRD command.

This 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.

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:

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).

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 v1.4b’) to the PC at the

new baud rate (followed by a carriage return) and

waits for a response.

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.

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.

Request for a new

baud rate divisor:

AT BRD hh

ELM327 responds

with ‘OK’

(if it is supported)

ELM327 sends

the AT I string

Program switches to

the new baud rate,

and waits for input

ELM327 waits

up to 75 msec*

for a carriage return

If the Rx is good,

Program sends a

carriage return

ELM327 says ‘OK’

(and remains at the

new baud setting)

Baud rate reverts

to the previous

baud rate

ELM327

received

yes

Print a prompt,

and wait for the

next command

ELM327 switches to

new baud rate and

waits for 75 msec*

ELM327P Datasheet(PDF) 60 Page - ELM Electronics

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