Bedienungsanleitung Elm 327 (Seite 5 von 94) (Englisch)

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Elm Electronics – Circuits for the Hobbyist

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ELM327DSJ

IgnMon / RTS (pin 15)

This input pin can serve one of two functions,

depending on how the Power Control options

(PP 0E) are set.

If both bit 7 and bit 2 of PP 0E are ‘1’s, this pin will

act as an Ignition Monitor. This will result in a switch

to the Low Power mode of operation, should the

IgnMon signal go to a low level, as would happen if

the vehicle’s ignition were turned off. An internal

‘debounce’ timer is used to ensure that the ELM327

does not shut down for noise at the input.

When the voltage at pin 15 is again restored to a

high level, and a time of 1 or 5 seconds (as set by

PP 0E bit 1) passes, the ELM327 will perform a

‘Warm Start’ and return to normal operation. A low to

high transition at pin 15 will in fact restore normal

operation, regardless of the setting of PP 0E bit 2, or

whether pin 15 was the initial cause for the low

power mode. This feature allows a system to control

how and when it switches to low power standby

operation, but still have automatic wakeup by the

ignition voltage, or even by a pushbutton.

If either bit 7 or bit 2 of PP 0E are ‘0’, this pin will

function as an active low ‘Request To Send’ input.

This can be used to interrupt the OBD processing in

order to send a new command, or as previously

mentioned, to highlight the fact that the ignition has

been turned off. Normally kept at a high level, this

input is brought low for attention, and should remain

so until the Busy line (pin 16) indicates that the

ELM327 is no longer busy, or until a prompt

character is received (if pin 16 is being used for

power control).

This input has Schmitt trigger wave shaping. By

default, pin 15 acts as the RTS interrupt input.

PwrCtrl / Busy (pin 16)

This output pin can serve one of two functions,

depending on how the Power Control options

(PP 0E) are set.

If bit 7 of PP 0E is a ‘1’ (the default), this pin will

function as a Power Control output. The normal state

of the pin will be as set by PP 0E bit 6, and the pin

will remain in that state until the ELM327 switches to

the Low Power mode of operation, when the output

changes to the opposite level. This output is typically

used to control enable inputs, but may also be used

for relay circuits, etc. with suitable buffering. The

discussion on page 84 (‘Modifications for Low Power

Standby Operation’) provides more detail on how to

use this output.

If bit 7 of PP 0E is a ‘0’, pin 16 will function as a

‘Busy’ output, showing when the ELM327 is actively

processing a command (the output will be at a high

level), or when it is idle, ready to receive commands

(the output will be low).

By default, bit 7 of PP 0E is ‘1’, so pin 16 provides

the Power Control function.

RS232Tx (pin 17)

This is the RS232 data transmit output. The signal

level is compatible with most interface ICs (the

output is high when idle), and there is sufficient

current drive to allow interfacing using only a PNP

transistor, if desired.

RS232Rx (pin 18)

This is the RS232 receive data input. The signal

level is compatible with most interface ICs (when at

idle, the level should be high), but can be used with

other interfaces as well, since the input has Schmitt

trigger wave shaping.

VSS (pin 19)

Circuit common must be connected to this pin.

VDD (pin 20)

This pin is the positive supply pin, and should always

be the most positive point in the circuit. Internal

circuitry connected to this pin is used to provide

power on reset of the ELM327 processor, so an

external reset signal is not required. Refer to the

Electrical Characteristics section for further

information.

ISO K (pin 21) and ISO L (pin 22)

These are the active high output signals which are

used to drive the ISO 9141 and ISO 14230 buses to

an active (dominant) level. Many new vehicles do not

require the L Line – if yours does not, you can simply

leave pin 22 open-circuited.

CAN Tx (pin 23) and CAN Rx (pin 24)

These are the two CAN interface signals that must

be connected to a CAN transceiver IC (see the

Pin Descriptions (continued)

ELM327

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