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ELM327

Using Higher RS232 Baud Rates

The RS232 serial interface has been maintained

throughout the ELM OBD products, largely due to its’

versatility. Older computers and PDAs can use it

directly, as can microprocessors. Newer computers

generally do not have RS232 ports, but they do have

USB and ethernet ports, both of which can be

converted to RS232 with a simple adapter.

Most people will build their ELM327 circuits with

an RS232 interface, mainly because it is relatively

easy and inexpensive to do so. A circuit such as the

one shown in Figure 9 requires very few components

and works extremely well at speeds up to 57600 bps.

Depending on your computer’s RS232 voltages, your

wiring practices, and your choice of components, it

may also work well at speeds as high as 115200 bps.

This is the approximate limit of such a circuit however,

and anything operating at this speed should be

thoroughly tested.

Users that would like to operate at speeds in the

range of 115200 bps (or higher) may wish to look at

some of the single IC solutions that are available.

These include

devices such as the ADM232A from

Analog

Devices

(http://www.analog.com/),

the

popular MAX232 series of ICs from Maxim Integrated

Products (http://www.maxim-ic.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 types of

devices are only rated for operation up to 120 Kbps,

however, so may not be suitable for higher data rates;

be sure to check the manufacturers data sheet before

committing to a design.

An RS232 interface needs relatively large voltage

swings, which are difficult to maintain at higher rates

with large cable capacitances to contend with. (A

typical interface is often limited to about 230400 bps

under ideal conditions.) If you need to operate the

ELM327 at these speeds or higher, it is recommended

that you consider alternatives.

One popular alternative is a USB data connection.

The USB interface is capable of very high data transfer

rates, certainly much higher than the 500 Kbps limit of

the ELM327. Several manufacturers offer special

‘bridge’ circuits that simplify connecting an RS232

device (such as the ELM327) directly to the USB bus.

Examples

are

the

CP2102

from

Silicon

Labs

(http://www.silabs.com/) or the FT232R from Future

Technology

Devices

(http://www.ftdichip.com/).

planning to use the higher baud rates (ie up to 500

Kbaud), these interfaces are essential.

We are often asked about the use of a direct

connection to microprocessor. That is certainly an

option, and one that allows a full speed connection at

essentially zero cost.

If you are developing such a

system, you can often simply connect the transmit

output from your microprocessor to the ELM327’s

receive, and the receive input to the ELM327’s

transmit. Provided that they share the same power

supplies, and you use good wiring practices between

them, the connection should work well. You do not

have to convert to RS232 voltage levels in order to use

the RS232 signals.

The default configuration for the ELM327 provides

an RS232 data rate of either 9600 baud, or 38400

baud, depending on the level at pin 6 during power up

or reset. While the 9600 baud rate is not adjustable,

the 38400 one is (beginning with v1.2 of the IC). There

are two ways that the rate can be changed – either

permanently with a Programmable Parameter, or

temporarily with an AT command.

Programmable Parameter ‘0C’ has been provided

to allow you to change the high speed baud rate to

what works best for you. You can change the value

several times if you wish, and each time that you do,

that new value becomes your power on default setting.

The type of storage used for this value is actually

called ‘non-volatile’, as it is not affected by power

cycles or resets. To change the setting requires only a

write to the ELM327’s EEPROM, using the AT PP

commands.

If, for example, you would like to have the IC use a

baud rate of 57.6K rather than the factory setting of

38.4K, simply change the value of PP 0C, and then

enable it. PP 0C is actually an internal divisor that is

stored in memory – the baud rate is calculated from

4000 divided by the value of the number that is stored.

To obtain a setting of 57.6, a baud rate divisor of 69 is

required (4000/69 is approximately 57.6). Since 69 in

decimal is 45 in hexadecimal, you tell the ELM327 to

set the value to 45:

>AT PP 0C SV 45

then enable the new value for use:

>AT PP 0C ON

from that point on (until PP 0C is turned off), the

ELM327 will communicate at the new baud rate.

If you are designing your own circuitry, it is

relatively easy to know what your circuit is capable of,

and then write a value to PP 0C once. Software

developers will not usually know what hardware is

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