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Elm 327 Manual

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Using Higher RS232 Baud Rates
A serial interface has always been provided with
our ELM OBD products, largely due to its versatility.
Older computers, microprocessors and PDAs can all
interface easily with it, as can USB, Bluetooth,
ethernet and WiFi modules. It is simply one of the
most versatile interfaces available.
Originally, our users almost exclusively used the
traditional RS232 interface to connect our integrated
circuits to their computers. Interface circuits were
easily made, or purchased, and could be used with a
very large range of devices. The large voltage swings
and cable capacitance worked against using the
interface for very high data rates, however, so we set
the default data rate of the ELM327 to a conservative
38.4 kbps.
If your application needs a traditional RS232
interface, then by all means use one. We offer a few
suggestions in the Example Applications section that
you might try. The discrete version that we show works
extremely well at speeds of up to 57600 bps, and
depending on several factors, it may also work well at
speeds as high as 115200 bps.
If you would like to operate your interface at
speeds of 115200 bps or higher, there are several
single IC solutions that are available. These include
devices such as the ADM232A from Analog Devices
(www.analog.com), or the popular MAX232 series of
integrated circuits from Maxim Integrated Products
(www.maximintegrated.com). These are all excellent
devices that can be used for speeds of up to 115.2
kbps. We do caution that many of these devices are
only rated for operation up to 120 kbps, however, so
may not be suitable for very high data rates - be sure
to check the manufacturers data sheet before
committing to a design.
An RS232 interface is typically limited to operating
at speeds of about 230.4 kbps maximum. If you wish
to go higher than that, then you must consider
alternatives - one of which is to use USB.
Almost all computers that are made today have
replaced the once familiar RS232 port with a USB one.
Software is readily available to make these look like
traditional serial (‘COM’) ports, and modules are
available for connecting to circuits like the ELM327.
Several manufacturers offer these modules (often
called ‘bridge’ circuits) – try the CP2102 from Silicon
Labs (www.silabs.com) or the FT232R from Future
Technology Devices (www.ftdichip.com). If planning to
use higher baud rates, these interfaces are essential.
We are often asked if it is possible to use a direct
connection to a microprocessor. That is certainly an
option, and one that allows a full speed connection at
essentially zero cost. If you are developing such an
interface, refer to page 76 for more details.
The default configuration for the ELM327 typically
provides a data rate of 38400 baud. There are two
ways that this rate can be changed – either
permanently with a Programmable Parameter, or
temporarily with an AT command.
Programmable Parameter ‘0C’ is the ELM327
device setting that stores the baud rate divisor. The
value is stored in ‘non-volatile’ memory (EEPROM)
that is not affected by power cycles or resets (but
changing this value may affect the operation of some
software packages, so be careful how you use it).
If you store a new value in PP 0C, then enable it,
your stored rate will become the new data rate after
the next reset. As an example, perhaps you would like
to have the ELM327 use a baud rate of 57.6 kbps,
rather than the factory setting of 38.4 kbps. To do this,
you need to determine the required value for PP 0C,
store this value in PP 0C, and then enable the PP.
The value stored in PP 0C is actually an internal
divisor that is used to determine the baud rate (the
baud rate in kbps is 4000 divided by the value of
PP 0C). To obtain a setting of 57.6 then, a baud rate
divisor of 69 is required (4000/69 is approximately
57.6). Since 69 in decimal is 45 in hexadecimal, you
need to tell the ELM327 to set the value of PP 0C to
45, with the set value command:
>AT PP 0C SV 45
then enable the new PP 0C value for use:
>AT PP 0C ON
from that point on, the default data rate will be 57.6K,
and not 38.4K. Note that the value that you write does
not become effective until the next full reset (a power
off/on, AT Z, or MCLR pulse).
If you are designing your own circuitry, you will
know what your circuit is capable of, and can assign a
baud rate with PP 0C. Software developers will not
usually know what hardware is to be connected,
however, so will not know what the limitations are. For
these users, 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. The chart shown here tries to
explain just how this command should be used.
ELM327
50 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
… 50 …


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