Example Applications

10 of 12

The circuit of Figure 2 (on the next page) shows

how the ELM630 might typically be used. While the

LIN standard does not specify the type of connector

that is to be used, it does specify that the physical

interface

will

essentially

be

an

enhanced

implementation of the ISO 9141 standard currently

used

in

automobiles.

It

provides

a

three

wire

connection, with power, data, and common.

The provision of a (nominal) 12V supply at the

connector provides a convenient means to power the

ELM630 circuit. This voltage is reduced to the required

5 volts by the 78L05 regulator shown, and filtered with

the capacitors. An LED provides visual feedback that

the supply is available.

The PNP transistor that is shown connected to the

LIN input (pin 13) serves mainly to level-shift the input

signal, and to invert it. A typical CMOS input will switch

state at about half the supply level (2.5V) which might

cause

problems

with

noise

in

an

automotive

environment. By providing a transistor buffer, the input

threshold is effectively raised to about 4V, increasing

noise immunity, while adding amplification, signal

clamping, and inversion. Note the use of the diode on

the

input,

which

prevents

possibly

damaging

backfeeds from entering the circuit, protecting the

transistor and the other components.

A very basic RS232 interface is shown connected

to pins 5 and 7 of the ELM630. 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 25 pin connector; if you are using a 9 pin, the

connections would be 2(RxD), 5(SG) and 3(TxD).

RS232

data

from

the

computer

is

directly

connected to pin 5 of the IC through only a 47K

Ω

current limiting resistor. This resistor allows for voltage

swings in excess of the supply levels while preventing

damage to the ELM630. A single 100K

Ω resistor is

also shown in this circuit so that pin 5 is not left floating

if the computer is disconnected. Note that pin 4 has

circuit for this type of connection.

Transmission of RS232 data is via the single PNP

transistor connected to pin 7. This transistor allows the

output voltage to swing between +5V and the negative

voltage stored on the 0.1µF capacitor (which is

charged by the computer’s TxD line). Although it is a

simple connection, it is quite effective for this type of

application.

Circuit timing is maintained by the crystal shown

ELM630

ELM630DSB

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

connected between pins 2 and 3. It is a common TV

type that can be easily and inexpensively obtained.

The 27pF crystal loading capacitors shown are only

typical, so you may have to select other values

depending on what is specified for the crystal you use.

setting power-up options as follows. Pins 8, 9, and 10

are all ‘0’ so the baud rate has been selected to be

‘auto’, allowing the IC to adapt to whatever baud rate

appears. Pin 11 is also low, meaning that the ‘Monitor

All’ mode will not be entered into on a power-up or

reset – the chip will simply issue a prompt character

and wait for instructions from the user. Finally, pin 12

for determining the baud rate will be 0.5 seconds. This

is often an adequate setting, even for the slowest

(1200 bps) systems.

Another application for the ELM630 is in the hand-

held “data snoop”, or “LIN Logic Probe” that is shown

in Figure 3. This circuit is very similar to that of Figure

causing the IC to immediately enter a “Monitor All”

mode on power-up. Setting pins 8, 9 and 10 to low

selects auto-baud operation, allowing this circuit to be

used on virtually all LIN systems. Note that since there

is no need to send commands to the ELM630 in this

case, the RS232 interface has been simplified even

further, eliminating the receive components. Power is

from three button cells, which provide 4.5V to the

circuit through a momentary action pushbutton switch.

The 4.5V is at the lower level of recommended

voltages, but is still within specified limits.

Hopefully this has provided enough information to

get you started with this convenient device. By

monitoring traffic on your automobile’s seat control

circuit, or your clothes dryer’s information bus, one can

learn a great deal. Certainly, you will be able to identify

what is normal, then of course you will know when it is

not…