ELM327P datasheet(47/51 Pages) ELM

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ELM327

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noise immunity), while R11 limits the current flow, and

R12 keeps Q4 off when the input is left open-circuited.

Resistor R36 has been added to the circuit of

Figure 9, to help turn transistor Q4 off more rapidly in

certain circumstances. The resistor is generally not

required, but it may be helpful if you are connected to

a very high capacitance J1850 VPW system then force

the ELM327 to operate in the J1850 PWM mode, and

experience some false BUS ERRORs. We show the

resistor as an option and leave the choice whether to

install it up to you.

The voltage monitoring circuitry for the AT RV

command is shown in this schematic connected to pin

2 of the ELM327. The two resistors simply divide the

battery voltage to a safe level for the ELM327, and the

capacitor filters out noise. As shipped, the ELM327

expects a resistor divider ratio as shown, and sets

nominal calibration constants assuming that. If your

application needs a different range of values, simply

choose the resistor values to maintain the input within

the specified 0-5V limit, and then perform an AT CV to

calibrate the ELM327 to your new divider ratio. The

maximum voltage that the ELM327 can show is 99.9V.

A very basic RS232 interface is shown connected

to pins 17 and 18 of the ELM327. This circuit ‘steals’

power from the host computer in order to provide a full

swing of the RS232 voltages without the need for a

negative supply. The RS232 pin connections shown

are for a standard 9 pin connector. If you are using a

25 pin one, you will need to compensate for the

differences. The polarity of the ELM327’s RS232 pins

is such that they are compatible with standard

interface ICs (MAX232, etc.), so if you should prefer

such an interface, you can remove all of the discrete

components shown and use one of those.

The four LEDs shown (on pins 25 to 28) have

been provided as a visual means of confirming circuit

activity. They are not essential, but it is nice to see the

visual feedback when experimenting.

Finally, the crystal shown connected between pins

9 and 10 is a standard 4.000MHz microprocessor type

crystal. The 27pF crystal loading capacitors shown are

typical only, and you may have to select other values

depending on what is specified for the crystal you

obtain. The crystal frequency is critical to circuit

operation and must not be altered.

We often receive requests for parts lists to

accompany our Example Applications circuits. Since

this circuit is more complex than most, we have

named/numbered all of the components and provided

a summary parts list (see Figure 10). Note that these

are only suggestions for parts. If you prefer another

LED colour, or have a different general purpose

transistor on hand, etc., by all means make the

change. A quick tip for those having trouble finding a

0.3” wide socket for the ELM327: many of the standard

14 pin sockets can be placed end-to-end to form one

0.3” wide 28 pin socket.

What if you only want to use one of the protocols?

What if you want to use a USB interface? These are

common questions that we receive, and both are

addressed in Figure 11.

There are a few single IC products on the market

that allow you to connect an RS232 system directly to

USB. We have shown the CP2102 by Silicon

Laboratories (http://www.silabs.com) in Figure 11, but

there are others available as well – Future Technology

Devices

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

for

example,

produces several. These ICs provide a very simple

and relatively inexpensive way to ‘bridge’ between

RS232 and USB, and as you can see, require very few

components to support them.

If using the CP2102, we do caution that it is very

small and difficult to solder by hand, so be prepared

for that. Also, if you provide protection on the data

lines with transient voltage suppressors (TVS’s), be

careful of which ones you choose, as some exhibit a

very large capacitance and will affect the transmission

of the USB data. Note also that the circuit as

presented will operate at a 38400 bits per second rate.

If you want to take full advantage of the speed of the

USB interface, you will need to change PP 0C.

Considering the OBD protocol portions of the

circuits in Figures 9 and 11, the differences should be

very apparent. The unused protocols in Figure 11 have

simply had their outputs ignored (left open circuit), and

their inputs wired to a convenient logic level (as CMOS

inputs must never be left floating).

The circuit maintains the status LEDs, and the

J1850 Bus+ circuitry, but the majority of the rest has

been eliminated. The voltage switching circuitry has

been reduced to a single 8V regulator as well, since

there will be no need to switch to 5V. Note that pin 3

has been intentionally left open-circuited as it is not

required by the voltage regulator.

The first time that this circuit is used, it will likely be

set to protocol 0, the default ‘automatic search’ mode

of operation (as shipped from the factory). When you

connect it to the J1850 VPW vehicle, it will first try a

J1850 PWM (protocol 1) connection, fail, and then

Example Applications (Cont’d)

ELM327P Datasheet(PDF) 47 Page - ELM Electronics

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