Bedienungsanleitung Elm 327 (Seite 50 von 94) (Englisch)

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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

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