ELM327P datasheet(43/51 Pages) ELM

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ELM327

Using Higher RS232 Baud Rates (Cont’d)

* Note that the 75msec time is adjustable

(with the AT BRT hh command)

connected to the ELM327 however, so will not know

what the limitations are. For these users, we have

provided the AT BRD command.

The AT BRD command allows a new baud rate 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 command. For

example, to try the 57600 baud rate that was

previously discussed, the controlling PC would send

the command:

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.

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 v1.2’) to the PC at the

new baud rate, and then 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, the baud rate then 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, so that

the ELM327 does not see a valid response. Your

software must check for the ‘OK’ response before

assuming that the new rate has been accepted.

Using this method, a program can quickly try

several baud rates, evaluating the connection each

time, and settling on the fastest suitable one. The new

baud rate will stay in effect until reset by an AT Z, or a

Power Off/On. It is not affected by the AT WS (Warm

Start) command.

Request for a new

baud rate divisor:

AT BRD hh

ELM327 responds

with ‘OK’

(if it is supported)

ELM327 waits for

75msec*, then sends

the AT I string

Program switches to

the new baud rate,

and waits for input

Wait up to 75 msec*

for a carriage return

character

If the Rx is good,

Program sends a

carriage return

Print ‘OK’

Revert to the

old baud rate

ELM327

received

yes

Print a prompt,

then wait for the

next command

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