ELM323 - OBD (ISO) to RS232 Interpreter - Description and Pin Descriptions

Oct 12, 2026

ELM323 is a 14-pin integrated circuit that converts OBD (On-Board Diagnostics) data in ISO formats into standard RS232 serial data, enabling PCs or PDAs to communicate with a vehicle through a serial port. It supports many ISO9141 and ISO14230 (KWP2000) protocols and can be customized via AT commands. The document outlines applications such as diagnostic trouble code readers and scan tools, then details the connection and block diagrams and each pin’s function, including crystal inputs, UART/RS232 I/O, LED drive outputs, and OBD bus interface signals.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages19

Reading Time48 min

AccessFree to download

Description Features

Applications

Connection Diagram and Block Diagram

Pin Descriptions

Power and Clock Pins

Serial and LED Output Pins

OBD Interface Pins

Ordering Information and Notices

ELM323 OBD (ISO) to RS232 Interpreter Description Features Since the 1996 model year, North American automobiles have been required to provide an OBD (On Board Diagnostics) data port for the connection of test equipment. This port is used to obtain emissions-related diagnostics information, and in some cases can also be used to obtain real-time vehicle operating parameters. The ELM323 is a 14 pin integrated circuit that, with only a few external components, is able to convert between the OBD data format and the standard RS232 serial data format. This allows virtually any personal computer or PDA to communicate with a vehicle using only a standard serial port and a terminal program. If desired, hobbyists can even create their own custom ‘scan tool’ by adding an interface program. Please note that while this integrated circuit has undergone significant changes recently, it is still considered to be an experimenter’s circuit. It does support a great many of the ISO9141 and ISO14230 (KWP2000) protocol vehicles, but not all of them, due to the many interpretations of these standards that are to be found in use. Applications • Diagnostic trouble code readers • Automotive scan tools • Teaching aids • Low power CMOS design • Crystal controlled for accuracy • ISO 9141-2 and ISO 14230-4 protocols • Configurable with AT commands • Standard ASCII character output • Four high current LED drive outputs | Connection Diagram PDIP and SOIC
(top view) | | | | --- | --- | --- | | VDD
XT1
XT2 LFmode
RS232Rx
RS232Tx
RSRx |

| VSS
OBDK
OBDL OBDIn
OBDTx
OBDRx
RSTx | 3.58 MHz Block Diagram XT1 3 XT2 ELM323DSC Elm Electronics – Circuits for the Hobbyist 1 of 19 < [http://www.elmelectronics.com/](http://www.elmelectronics.com/) > ELM323 Pin Descriptions VDD (pin 1) 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 microprocessor, so an external reset signal is not required. Refer to the Electrical Characteristics section for further information. XT1 (pin 2) and XT2 (pin 3) A 3.579545 MHz NTSC television colourburst crystal is connected between these two pins. Crystal loading capacitors (typically 27pF) will also normally be connected between each of these pins and the circuit common (Vss) . LFmode (pin 4) This input is used to select the default linefeed mode after a power-up or system reset. If it is at a high level, then by default messages sent by the ELM323 will be terminated with both a carriage return and alinefeed character. If it is at a low level, lines will be terminated by a carriage return only. This behavior can always be modified by issuing AT L0 or AT L1 commands (see the section on AT Commands) . RS232Rx (pin5) A computer’s RS232 transmit signal can be directly connected to this pin from the RS232 line as long asa current limiting resistor (typically about 47KΩ ) is installed in series. (Internal protection diodes will pass the resistor current safely to the supply connections, protecting the ELM323 . ) Internal signal inversion and Schmitt trigger waveshaping provide the necessary signal conditioning. RS232Tx (pin 6) This is the RS232 transmit or data output pin. The signal level is compatible with most interface ICs, and there is sufficient current drive to allow interfacing using only a PNP transistor, if desired. LED Drive Outputs (pins 7, 8, 9, and 10) These four pins are driven to low levels when the ELM323 is transmitting or receiving RS232 or OBD data. Otherwise, they are at a high level. Current capability is suitable for directly driving most LEDs through current limiting resistors. If unused, these pins should be left open-circuited. OBDIn (pin11) The OBD data is input to this pin, with a high logic level representing the active state of the OBD K line. No Schmitt trigger input is prov

FAQs

What does the ELM323 convert between, and why is it useful?

It converts OBD data from ISO formats into standard RS232 serial data. This allows a personal computer or PDA to communicate with a vehicle using a serial port and terminal program.

Which vehicle communication protocols does the ELM323 support?

It supports many ISO9141 and ISO14230 (KWP2000) protocol vehicles, but not all implementations due to variations in how standards are interpreted.

How do the LFmode pin and AT commands affect message termination?

LFmode selects the default linefeed behavior after reset. If high, messages terminate with carriage return plus linefeed; if low, they terminate with carriage return only, and this can be modified using AT L0 or AT L1 commands.

Oct 12, 2026

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