Bedienungsanleitung Elm 327 (Seite 79 von 94) (Englisch)

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what you are doing while testing.

The final OBD interface shown is in the bottom left

corner, and is used for the two J1850 standards. The

J1850 VPW standard needs a positive supply of up to

8V while the J1850 PWM needs 5V, so we have

shown a two level supply that can provide both. This

dual voltage supply uses a 317L adjustable regulator

as shown, controlled by the pin 3 output. With the

resistor values given, the selected voltages will be

about 8.0V and 5.5V, which works well for most

vehicles. Note that the 317L is able to maintain

regulation with a minimum of 1.5 mA of current, so we

have used a 470Ω resistor between the output and

adjust pins. The larger 317 regulator typically requires

3.5 mA so would need the resistors scaled down

proportionally. Once suitable voltages have been

generated, they are driven by the Q1-Q2 combination

for the Bus+, and Q3 for the Bus-.

The J1850 VPW input uses a resistor divider,

similar to that which was used for the ISO input.

Typical threshold voltages with the resistors shown will

be about 4.2V (rising) and 2.2V (falling). The J1850

PWM input is a little different in that it must convert a

differential input into a single-ended one for use by the

ELM327. This is done by connecting Q4 across the

input so that it operates as a difference amplifier. The

Q4-D3 series combination sets a threshold voltage of

about 1V (for improved noise immunity), while R11

limits the current flow, and R12 makes sure that Q4 is

off when it should be. The circuit works well as shown,

but the R14 passive pullup time constant can be easily

affected by stray capacitance - be aware of this if

connecting test equipment to pin 13.

Resistor R10 is the final J1850 component. We

added this to help discharge the data bus more rapidly

when it was found that some vehicles showed higher

capacitance than others. The resistor may not be

required for many vehicles - the choice is yours. If you

should see reports of BUS ERRORs with a J1850

vehicle, it may be this capacitance causing problems

(you will need to ‘scope the signal to be sure).

Moving on, the R25-R26 voltage divider shown

connected to pin 2 is used for the vehicle voltage

monitoring circuitry. The two resistors simply divide the

battery voltage to a safer level for the ELM327, and

the capacitor (C2) helps to filter 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 your resistor values to maintain the

input within the ELM327’s VSS to VDD voltage range,

and then perform an AT CV to calibrate the ELM327 to

your new ratio. The maximum voltage that the ELM327

can show is 99.9V (it’s a software limit, not hardware).

Four LEDs are shown on pins 25 to 28. These

have been provided as a visual means of confirming

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

see the visual feedback.

On the right side of the circuit, the ELM327’s

RS232 pins (17 and 18) are shown connected to an

FTDI USB module. This module makes it very easy to

connect the ELM327 circuit to your computer - all you

need is the VCP Driver software, which is available for

free from the FTDI web site (www.ftdichip.com). The

module pinout matches a 9 pin D-sub connector, so

you can simply solder it in where the RS232 circuitry

used to go. Diode D5 and resistor R32 have been

added to the interface to prevent the USB supply from

backfeeding into the ELM327.

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 that you

use. 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

and numbered all of the components and provided a

summary parts list (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., then 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. For more help with building and

testing the circuit, see our ‘AN02 - ELM327 Circuit

Construction’ application note.

What if you only want to support one of the

ELM327’s protocols? Well, you may do that if you

wish. Simply remove the OBD interfaces that you do

not require, and connect the rest. Since you must

never leave a CMOS input floating (open-circuited),

you will need to add a jumper or two on the unused

inputs. See the ‘Unused Pins’ section for advice on

what to connect the pins to.

Some people still prefer to interface their circuits

with standard RS232 circuitry. For these, we offer the

Example Applications (continued)

79 of 94

ELM327

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Elm 327 Seite 79

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