Specifications

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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.

• 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

• Diagnostic trouble code readers

• Automotive scan tools

• Teaching aids

Description

Applications

Block Diagram

Features

Connection Diagram

PDIP and SOIC

(top view)

VDD VSS

OBDK

LFmode

XT1

XT2

OBDIn

OBDTx

RSRx

OBDRx

RS232Tx

OBDL

RSTx

RS232Rx

XT1 XT2

Timing and

Control

Interpreter

OBD

Interface

OBDK

OBDIn

OBDL

RS232Tx

RS232Rx

LFmode

OBDTx

OBDRx

3.58 MHz

RS232

Interface

RSRx

RSTx

Elm Electronics – Circuits for the Hobbyist

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OBD (ISO) to RS232 Interpreter

1 of 19ELM323DSD

ELM323

Summary of content (19 pages)

PAGE 1
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.
PAGE 2
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.
PAGE 3
ELM323 Absolute Maximum Ratings Storage Temperature....................... -65°C to +150°C Ambient Temperature with Power Applied....................................-40°C to +85°C Voltage on VDD with respect to VSS............ 0 to +7.0V Note: Stresses beyond those listed here will likely damage the device. These values are given as a design guideline only. The ability to operate to these levels is neither inferred nor recommended. Voltage on any other pin with respect to VSS...........................
PAGE 4
ELM323 Overview The following describes how to use the ELM323 to obtain a great deal of information from your vehicle. To some, the following information will be overwhelming, and for others it will not be nearly enough. We begin by discussing just how to talk to the IC using a PC, then go on to explain how to change options using ‘AT’ commands, and finally go on to actually use the ELM323 to obtain trouble codes (and reset them).
PAGE 5
ELM323 AT Commands Several parameters within the ELM323 can be adjusted in order to modify its behaviour. These do not normally have to be changed before attempting to talk to the vehicle, but occasionally the user may wish to customize the settings; for example by turning the character echo off, adjusting a timeout value, or changing the header bytes. In order to do this, internal ‘AT’ commands must be issued.
PAGE 6
ELM323 AT Commands (continued) E0 and E1 [ Echo off (0) or on(1) ] These commands control whether or not characters received on the RS232 port are retransmitted (or echoed) back to the host computer. To reduce traffic on the RS232 bus, users may wish to turn echoing off by issuing ATE0. The default is E1 (echo on). FD [ send Formatted Data ] This command requests that all of the vehicle’s responses be returned as standard ASCII characters (which are readable with virtually all terminal programs).
PAGE 7
ELM323 AT Commands (continued) response bytes that were received from the vehicle. There will be no added space characters, and no trailing carriage returns or linefeed characters inserted between messages. This provides a very compact format for data transfer. Note that the length byte only represents the total number of data bytes following, and does not include itself.
PAGE 8
ELM323 AT Commands Summary hexadecimal value from 00 to FF. The maximum possible time delay of 5.10 seconds thus occurs when a value of FF (or decimal 255) is used. The default setting is 7D (or decimal 125) which provides a nominal delay of 2.5 seconds between messages. Note that the value 00 (zero) is treated as a very special case, and must be used with caution. It is used to stop all periodic messages, by telling the ELM323 internally that the bus is no longer active.
PAGE 9
ELM323 Bus Initiation Both the ISO 9141-2 and ISO 14230-4 (KWP2000) standards require that the vehicle’s OBD bus be initialized before any communications can take place. The ISO 9141 standard allows for only a slow (2 to 3 second) process, while ISO 14230 allows for both the slow method, and a faster alternative. In either case, once the bus has been initiated, communications must take place at least once every five seconds, or the bus will revert to a low-power ‘sleep’ mode.
PAGE 10
ELM323 OBD Commands If the bytes received on the RS232 bus do not begin with the letters ‘A’ and ‘T’, they are assumed to be OBD commands for the vehicle. Each pair of ASCII bytes will be tested to ensure that they are valid hexadecimal digits, and will then be combined into single data bytes for transmitting to the vehicle. OBD commands are actually sent to the vehicle embedded in a data packet.
PAGE 11
ELM323 Talking to the Vehicle The ELM323 cannot be directly connected to a vehicle as it is, but needs support circuitry as shown in the Example Applications section. Once incorporated into such a circuit, one need only use a terminal program to send bytes to and receive them from the vehicle via the ELM323. SAE standards specify that each group of bytes sent to the vehicle must adhere to a set format.
PAGE 12
ELM323 Interpreting Trouble Codes Likely the most common use that the ELM323 will be put to is in obtaining the current Diagnostic Trouble Codes or DTCs. Minimally, this requires that a mode 03 request be made, but first one should determine how many trouble codes are presently stored.
PAGE 13
ELM323 Resetting Trouble Codes The ELM323 is quite capable of resetting diagnostic trouble codes, as this only requires issuing a mode 04 command. The consequences should always be considered before sending it, however, as more than the MIL (or ‘Check Engine’ lamp) will be reset.
PAGE 14
ELM323 Monitoring the Bus Some vehicles use the OBD bus for information transfer during normal vehicle operation, passing a great deal of information over it. A lot can be learned if you have the good fortune to connect to one of these vehicles, and are able to decipher the contents of the messages. Other vehicles cannot be initialized, and instead continually send information; the only way to read the data from them is by monitoring everything that is being sent, and extracting the useful data.
PAGE 15
ELM323 Advanced Data Retrieval – Setting the Headers Prior to v2.0, the ELM323 used a fixed format for the message headers, so that only one type of data (the mandated emissions-related information) could be retrieved. Starting with v2.0, the header bytes can be specified by the user, allowing for the direct retrieval of a great deal more data. Note that only the OBDII diagnostic codes have been mandated, so there is no requirement for all vehicles to support these extra capabilities, and some do not.
PAGE 16
ELM323 Quick Guide for Reading Trouble Codes If you don’t use your ELM323 for some time, this entire data sheet may seem like quite a bit to review if your ‘Check Engine’ light does eventually come on. The following provides a quick procedure which may prove helpful in that case (note that the ‘>’ is the ELM323’s prompt character): Connect using HyperTerminal, ZTerm, etc.
PAGE 17
ELM323 Example Applications The SAE J1962 standard dictates that all OBD compliant vehicles must provide a standard connector near the driver’s seat, the shape and pinout of which is shown in Figure 4 below. The circuitry described here can be used to connect to this J1962 plug without modification to your vehicle.
PAGE 18
ELM323 OBD Interface 78L05 16 (Battery Positive) +5V 0.01µF 0.1µF 750Ω 5 (Signal Ground) Notes: - NPN transistors are 2N3904 or similar - PNP transistors are 2N3906 or similar - Diodes are 1N4148, 1N4001, etc. ‘Power On’ LED 510Ω 15 (L Line) 2.2KΩ +5V +5V 510Ω (K Line) 7 330Ω 330Ω 2.2KΩ Status LEDs 14 +5V 13 12 11 10 9 +5V RS232 Interface (DB9F) 8 10KΩ 323 10KΩ 2 (RxD) +5V 10KΩ 1 2 3 4 5 6 7 4.7KΩ 0.1µF 5 (SG) 3.58MHz 4.
PAGE 19
ELM323 Example Applications (continued) As a final example, we provide an OBD monitor. There are times when it would be convenient to be able to simply monitor the OBD bus for one reason or another – personal learning, monitoring others in teaching environments, and also there are apparently some vehicles produced that continually send OBD information, so can not be ‘read’ in the standard way. For these situations, a simplified version of the circuit in Figure 5 can be used as shown in Figure 6 below.