ELM327 OBD to RS232 Interpreter Description Features Almost all of the automobiles produced today • Power Control with standby mode are required, by law, to provide an interface for the • Universal serial (RS232) interface connection of diagnostic test equipment. The data transfer on these interfaces follow several standards, • Automatically searches for protocols but none of them are directly usable by PCs or smart • Fully configurable with AT commands devices. The ELM327 is designed to act as a bridge between these On-Board Diagnostics (OBD) ports • Low power CMOS design and a standard RS232 serial interface. In addition to being able to automatically detect and interpret nine OBD protocols, the ELM327 also Connection Diagram provides support for high speed communications, a PDIP and SOIC low power sleep mode, and the J1939 truck and bus (top view) standard. It is also completely customizable, should you wish to alter it to more closely suit your needs. MCLR OBD Tx LED The following pages discuss all of the ELM327’s Vmeasure OBD Rx LED features in detail, how to use it and configure it, as J1850 Volts RS232 Tx LED well as providing some background information on J1850 Bus+ RS232 Rx LED the protocols that are supported. There are also Memory CAN Rx schematic diagrams and tips to help you to interface Baud Rate CAN Tx to microprocessors, construct a basic scan tool, and LFmode ISO L to use the low power mode. VSS ISO K XT1 VDD Applications XT2 VSS • Diagnostic trouble code readers VPW In RS232 Rx ISO In RS232 Tx • Automotive scan tools PWM In PwrCtrl / Busy • Teaching aids J1850 Bus- IgnMon / RTS Block Diagram 4.00 MHz Memory MCLR Baud Rate 6 XT19 10 XT2 5 1 LFmode 7 Vmeasure 2 A/D RS232Rx 18 RS232 Command Converter Interface RS232Tx 17 and Protocol Interpreter ISO 15765-4 ISO 9141-2 SAE J1850 SAE J1939 ISO 14230-4 PWM & VPW PwrCtrl / Busy 16 Power IgnMon / RTS 15 Control 23 24 21 22 12 3 4 14 13 11 25 26 27 28 OBD interfaces status LEDs ELM327DSJ Elm Electronics – Circuits for the Hobbyist 1 of 94 www.elmelectronics.com ELM327 Contents The Basics Description................................................................................... 1 Features.......................................................................................1 Applications..................................................................................1 Block Diagram..............................................................................1 Connection Diagram.................................................................... 1 Pin Descriptions........................................................................... 4 Unused Pins.................................................................................6 Absolute Maximum Ratings......................................................... 6 Electrical Characteristics..............................................................7 Using the ELM327 Overview...................................................................................... 8 Communicating with the ELM327................................................ 8 AT Commands........................................................................... 10 AT Command Summary.............................................................10 AT Command Descriptions........................................................ 12 Reading the Battery Voltage...................................................... 29 OBD Commands........................................................................30 Talking to the Vehicle.................................................................31 Bus Initiation...............................................................................33 Interpreting Trouble Codes........................................................ 34 Resetting Trouble Codes........................................................... 35 Quick Guide for Reading Trouble Codes................................... 35 Selecting Protocols.................................................................... 36 OBD Message Formats..............................................................38 Setting the Headers................................................................... 39 Multiline Responses...................................................................42 CAN Message Types................................................................. 44 Multiple PID Requests................................................................45 Response Pending Messages................................................... 45 CAN Receive Filtering - the CRA command.............................. 46 Using the CAN Mask and Filter..................................................47 Monitoring the Bus..................................................................... 48 Restoring Order..........................................................................49 Advanced Features Using Higher RS232 Baud Rates...............................................50 Setting Timeouts - the AT ST and AT AT Commands............... 52 SAE J1939 Messages................................................................53 Using J1939............................................................................... 55 The FMS Standard.....................................................................58 The NMEA 2000 Standard.........................................................59 Periodic (Wakeup) Messages.................................................... 59 Altering Flow Control Messages................................................ 60 Using CAN Extended Addresses............................................... 61 CAN Input Frequency Matching.................................................62 ELM327DSJ Elm Electronics – Circuits for the Hobbyist 2 of 94 www.elmelectronics.com ELM327 Contents Advanced Features Programming Serial Numbers....................................................63 (continued) Saving a Data Byte.................................................................... 63 The Activity Monitor....................................................................64 Power Control............................................................................ 64 Programmable Parameters........................................................68 Programmable Parameter Summary......................................... 69 Design Discussions Maximum CAN Data Rates........................................................ 74 Microprocessor Interfaces..........................................................76 Upgrading Versions....................................................................77 Example Applications.................................................................78 Figure 9 - An OBD to USB Interpreter................................. 80 Figure 10 - Parts List for Figure 9........................................81 Figure 11 - A Low Speed RS232 Interface..........................81 Figure 12 - A High Speed RS232 Interface......................... 82 Figure 13 - An Alternative USB Interface............................ 82 Figure 14 - Connecting to a 3.3V System............................83 Modifications for Low Power Standby Operation....................... 84 Misc. Information Error Messages and Alerts.........................................................87 Version History...........................................................................90 Outline Diagrams....................................................................... 92 Ordering Information.................................................................. 92 Copyright and Disclaimer...........................................................92 Index.......................................................................................... 93 ELM327DSJ Elm Electronics – Circuits for the Hobbyist 3 of 94 www.elmelectronics.com ELM327 Pin Descriptions MCLR (pin 1) A momentary (>2µsec) logic low applied to this input return and a linefeed character. If it is at a low level, will reset the ELM327. If unused, this pin should be lines will be terminated by a carriage return only. connected to a logic high (VDD) level. This behaviour can always be modified by issuing an AT L1 or AT L0 command. Vmeasure (pin 2) This analog input is used to measure a 0 to 5V VSS (pin 8) signal that is applied to it. Care must be taken to Circuit common must be connected to this pin. prevent the voltage from going outside of the supply levels of the ELM327, or damage may occur. If it is XT1 (pin 9) and XT2 (pin 10) not used, this pin should be tied to either VDD or VSS. A 4.000 MHz oscillator crystal is connected between these two pins. Loading capacitors as required by J1850 Volts (pin 3) the crystal (typically 27pF each) will also need to be This output can be used to control a voltage supply connected between each of these pins and circuit for the J1850 Bus+ output. The pin normally outputs common (Vss). a logic high level when a nominal 8V is required (for Note that this device has not been configured for J1850 VPW), and a low level for 5V (for J1850 operation with an external oscillator – it expects a PWM), but this can be changed with PP 12. If this crystal to be connected to these pins. Use of an switching capability is not required for your external clock source is not recommended. Also, application, this output can be left open-circuited. note that this oscillator is turned off when in the Low Power or ‘standby’ mode of operation. J1850 Bus+ (pin 4) This active high output is used to drive the VPW In