elm323 elm electronics ?circuits for the hobbyist < http://www.elmelectronics.com/ > obd (iso) to rs232 interpreter since the 1996 model year, north american automobiles have been required to provide an obd, or on board diagnostics, port for the connection of test equipment. data is transferred serially between the vehicle and the external equipment using these connections, in a manner specified by the society of automotive engineers (sae). these are serial connections, but as the voltage levels and the protocols differ, computer serial ports cannot be directly used to communicate with vehicles. the elm323 is an 14 pin integrated circuit that, with only a few external components, is able to provide the interface, interpreting the obd signals and reformatting them as standard ascii characters. 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 create their own custom ?can tool?by adding an interface program. this integrated circuit was designed to provide a cost-effective way for experimenters to work with an obd system, so many features such as rs232 handshaking, variable baud rates, etc., have not been implemented. in addition, this device is only able to communicate using the 10.4khz iso 9141 protocol that is commonly used by daimler chrysler, and many ?orld built?vehicles. low power cmos design crystal controlled for accuracy configurable with at commands standard ascii character output four high current led drive outputs 10.4khz iso 9141 protocol diagnostic trouble code readers automotive scan tools description applications block diagram 1 of 11 features elm323dsa connection diagram pdip and soic (top view) v dd v ss obdk lfmode
elm323 elm electronics ?circuits for the hobbyist < http://www.elmelectronics.com/ > pin descriptions 2 of 11 all rights reserved. copyright 2001 elm electronics. every effort is made to verify the accuracy of information provided in this document, but no representation or warranty can be given and no liability assumed by elm electronics with respect to the accuracy and/or use of any products or information described in this document. elm electronics will not be responsible for any patent infringements arising from the use of these products or information, and does not authorize or warrant the use of any elm electronics product in life support devices and/or systems. elm electronics reserves the right to make changes to the device(s) described in this document in order to improve reliability, function, or design. v dd (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 3.579545mhz ntsc television colourburst crystal is connected between these two pins. crystal loading capacitors (typically 27pf) will also normally be connected between each of the pins and the circuit common (vss). lfmode (pin 4) this input is used to select the default linefeed mode after a powerup or system reset. if it is at a high level, then by default lines sent by the elm323 will be terminated with both a carriage return and a linefeed character. if it is at a low level, lines will be terminated by a carriage return only. this behavior can still be modified by issuing atl0 or atl1 commands (see the section on at commands). rs232rx (pin5) a computer? rs232 transmit signal can be directly connected to this pin from the rs232 line as long as a current limiting resistor (typically about 47k w ) is installed in series. (internal protection diodes will pass the input currents 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, and are otherwise at a high level. current capability is suitable for directly driving most leds through current limiting resistors. 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 provided, so the obd signal should be buffered to minimize transition times for the internal cmos circuitry. the external level shifting circuitry used usually accomplishes this. obdl (pin 12) and obdk (pin 13) these are the active high output signals which are used to drive the obd bus, using external npn transistors. data transfer normally occurs only by the k line, but the standards require that the l line be implemented as well in order to properly initialize the bus. see the example application section for more details. v ss (pin 14) circuit common is connected to this pin. this is the most negative point in the circuit. elm323dsa
elm electronics ?circuits for the hobbyist < http://www.elmelectronics.com/ > elm323 electrical characteristics absolute maximum ratings storage temperature....................... -65? to +150? ambient temperature with power applied.................................... -40? to +85? voltage on v dd with respect to v ss ............ 0 to +7.0v voltage on any other pin with respect to v ss ........................... -0.6v to (v dd + 0.6v) 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. 3 of 11 notes: 1. this integrated circuit is produced with a microchip technology inc.? pic16c505 as the core embedded microcontroller. for further device specifications, and possibly clarification of those given, please refer to the appropriate microchip documentation (available at http://www.microchip.com/). 2. this spec must be met in order to ensure that a correct power on reset occurs. it is quite easily achieved using most common types of supplies, but may be violated if one uses a slowly varying supply voltage, as may be obtained through direct connection to solar cells, or some charge pump circuits. 3. device only. does not include any load currents. 4. this specification represents the current flowing through the protection diodes when applying large voltages to the rs232rx input (pin 5) through a current limiting resistance. currents quoted are the maximum that should be allowed to flow continuously. 5. nominal data transfer rate when the recommended 3.58 mhz crystal is used as the frequency reference. data is transferred to and from the elm323 with 8 data bits, no parity, and 1 stop bit (8 n 1). elm323dsa all values are for operation at 25? and a 5v supply, unless otherwise noted. for further information, refer to note 1 below. characteristic minimum typical maximum conditions units supply voltage, v dd 4.5 5.0 5.5 v v dd rate of rise 0.05 v/ms average supply current, i dd 1.0 2.4 ma input low voltage v ss 0.15 x v dd v input high voltage v dd v 0.85 x v dd output low voltage 0.6 v output high voltage v v dd - 0.7 current (sink) = 8.7ma current (source) = 5.4ma see note 2 rs232rx pin input current ma see note 4 -0.5 rs232 baud rate baud see note 5 9600 +0.5 see note 3
4 of 11 elm323 elm323dsa elm electronics ?circuits for the hobbyist < http://www.elmelectronics.com/ > communicating with the elm323 the elm323 relies on a standard rs232 type serial connection to communicate with the user. the data rate is fixed at 9600 baud, with 8 data bits, no parity bit, 1 stop bit, and no handshaking (often referred to as 9600 8n1). all responses from the ic are terminated with a single carriage return character, and optionally a line feed character as well. make sure your software is configured properly for the mode you have chosen. properly connected and powered, the elm323 will energize the four led outputs in sequence (as a ?amp test? and will then send the message: elm323 v1.0 > in addition to identifying the version of the ic, receipt of this string is a convenient way to be sure that the computer connections and the settings are correct. however, at this point no communications have taken place with the vehicle, so the state of that connection is still unknown. the ??character displayed above is the elm323? prompt character. it indicates that the device is in its idle state, ready to receive characters on the rs232 port. messages sent from the computer can either be intended for the elm323? internal use, or for reformatting and passing on to the obd bus. the elm323 can quickly determine where the received characters are to be directed, by analyzing the entire string once the entire message is received. commands for the elm323? internal use will always begin with the characters ?t?(as is common with modems), while commands for the obd bus are only allowed to contain the ascii codes for hexadecimal digits (0 to 9 and a to f). whether an ?t?type internal command or a hex string for the obd bus, all messages to the elm323 ordering information these integrated circuits are available in either the 300 mil plastic dip format, or in the 150 mil soic surface mount type of package. to order, add the appropriate suffix to the part number: 300 mil plastic dip................................... elm323p 150 mil soic........................................ ELM323SM must be terminated with a carriage return character (hex ?d? before it will be acted upon. the one exception is when an incomplete string is sent and no carriage return appears. in this case, an internal timer will automatically abort the incomplete message after about 10 seconds, and the elm323 will print a single question mark to show that the input was not understood (and was not acted upon). messages that are not understood by the elm323 (syntax errors) will always be signalled by a single question mark (??. these include incomplete messages, invalid at commands, or invalid hexadecimal digit strings, but are not an indication of whether or not the message was understood by the vehicle. one must keep in mind that the elm323 is a protocol interpreter that makes no attempt to assess the obd messages for validity ?it only ensures that an even number of hex digits were received, combined into bytes, and sent out the obd port, and it does not know if the message sent to the vehicle is in error. incomplete or misunderstood messages can also occur if the controlling computer attempts to write to the elm323 before it is ready to accept the next command (as there are no handshaking signals to control the data flow). to avoid a data overrun, users should always wait for the prompt character (?? before issuing the next command. finally, a few convenience items to note. the elm323 is not case-sensitive, so ?tz?is equivalent to ?tz? and to ?tz? the device ignores space characters as well as control characters (tab, linefeed, etc.) in the input, so they can be inserted anywhere to improve readability and, finally, issuing only a single carriage return character will repeat the last command received (making it easier to request updates on dynamic data such as engine rpm).
5 of 11 elm323 elm323dsa elm electronics ?circuits for the hobbyist < http://www.elmelectronics.com/ > at commands the elm323 accepts internal configuration commands in much the same manner that modems do. any message received at any time, that begins with the character ??followed by the character ??will be considered an internal configuration or ?t command. these are executed upon receipt of the terminating carriage return character, and completion of the command is acknowledged by the printing of the characters ?k? communications on the obd bus can generally begin without requiring the issuance of any at commands, as the factory default settings should be appropriate for most applications. occasionally the user may wish to customize settings, such as turning the character echo off. in these cases, at commands must be issued. the following is a summary of the at commands that are recognized by the current version of the elm323. note that they are not case-sensitive, and that the character ??is the number ?ero? atd, atd0 and atd1 all three of these commands provide the same function ?to reset the e, h and l options to their default (or factory) settings. this is equivalent to issuing an ate1, an ath0, and also an atl1 or atl0. (the level at lfmode is read whenever this command is issued, and l is set appropriately). ate0 and ate1 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. default is e1 (echo on). ath0 and ath1 these strings control whether or not the header information is shown in the responses. all obd messages have an initial (header) string of three bytes and a trailing check digit that is normally not displayed by the elm323. to see this extra information, users can turn the headers on by issuing an ath1. the default is h0 (headers off). ati issuing this command causes the chip to identify itself, by printing the startup product id string (this is currently ?lm 323 v1.0?. software can use this to determine exactly which integrated circuit it is talking to, without resorting to resetting the entire ic. atl0 and atl1 whether the elm323 transmits a linefeed character whenever a carriage return character is sent is controlled by this option. if an atl1 is issued, linefeed generation will be turned on, and for atl0, it will be off. users may wish to have this option on if using a terminal program, but off if using a custom interface (as the extra characters transmitted will only serve to slow the vehicle polling down). the default setting is determined by the logic level at the lfmode pin on powerup or reset ?if a ??or high level, then the default is l1, otherwise it is l0. atz this combination causes the chip to perform a complete reset as if power were cycled off and then on again. all settings are returned to their default values, and the chip will be put in the idle state, waiting for characters on the rs232 bus. bus initiation both the iso 9141 and iso 14230 (kwp2000) standards require that the vehicle? bus be initialized before communications can take place. iso 9141 allows for only a slow (2 to 3 second) process while iso 14230 allows this as well as a faster alternative. in either case, once the bus has been initiated, communications must take place at least once every five seconds, to keep it ?live? the elm323 takes care of this bus initiation and maintenance for you. when you first request data from the vehicle, you will be presented with the following message: bus init: ...ok an attempt is initially made to use the fast initiation process, and if it is successful, you will not see the
6 of 11 elm323 elm323dsa elm electronics ?circuits for the hobbyist < http://www.elmelectronics.com/ > obd commands if the bytes received on the rs232 bus do not begin with the letters a and t, they are assumed to be commands for the vehicle? obd bus. the bytes will then be tested to ensure that they are valid pairs of hexadecimal digits, and will be combined into bytes for transmitting. recall that no checks are made as to the validity of the obd command ?data is simply retransmitted as received. obd commands are actually sent to the vehicle embedded in a data packet. the standards require that three header bytes and an error checksum be included in every message, and the elm323 inserts them automatically for the user (they never change in value, so are stored internally). to view the extra bytes that are received with the vehicle? messages, issue an ath1 internal command. occasionally vehicles will have more than one module responding to a request, and it may be useful to show which one responded. to do this, turn the headers on and note the third byte of the response - this is the address of the sender. most obd commands to the vehicle are one or two bytes in length, but some can be three or more bytes long. the elm323 is capable of sending seven data bytes (14 hexidecimal digits), the maximum number allowed by the standards. attempts to send either an odd number of digits or too many digits will result in a syntax error ?the entire command is then ignored and a single question mark printed. hexadecimal digits are used for all of the data exchange with the elm323 because it is the data format used in the relevant sae standards. it is consistent with mode request listings and is the most frequently used format used to display results. with a little practice, it should not be very difficult to deal in hex numbers, but some people may want to obtain a conversion table or keep a calculator nearby. all users will be required to manipulate the results in some way, though (combine bytes and divide by 4 to obtain rpm, divide by 2 to obtain degrees of advance, etc.), and may find a software front-end helpful. as an example of sending a command to the vehicle, assume that a6 (or decimal 166) is the command that is required to be sent. in this case, the user would type the letter a, then the number 6, then would press the return key. these three characters would be sent to the elm323 on the rs232 bus. the elm323 would store the characters as they are received, and when the third character (the carriage return) is received, begin to assess the other two. it would see that they are both valid hex digits, and would convert them to a one byte value (decimal value is 166). four header bytes would be added, and a total of five bytes would be sent to the vehicle. note that the carriage return character is only a signal to the elm323, and is never sent to the vehicle. after sending a command, the elm323 listens on the obd bus for any responses that are directed to it. each received byte is converted to the equivalent hexadecimal pair of ascii characters and transmitted on the rs232 port for the user. rather than send control characters which are unprintable on most terminals, the digits are sent as numbers and letters (eg. the hex digit ??is transmitted as decimal value 65, and not 10). if there was no response from the vehicle, due to no data being available, or because the command is not supported, a ?o data?message will be sent. see the error messages section for a description of this message and others. bus initiation (continued) three dots appear, as they are only printed as the slow initiation is taking place. if either initiation method is succesful, you will see the ?k?printed as shown, and on the next line the response to your request, but if they all failed, only an error message will appear. (the most common error is in forgetting to turn the vehicle? key to ?n?before attempting to talk to the vehicle.) once initiated, the elm323 does what is required to keep the bus alive, without any intervention from the user. (if you have installed leds, you will see ?ummy messages being sent every few seconds in order to create bus activity.) since rs232 input has the highest priority within the elm323, it is conceivable that a combination of erronous inputs could cause the chip to miss a wakeup call, but this is not very likely. as a final note, the elm323 does include some code for iso 14230 initiation and data transfer, but at this time we cannot guarantee compliance until it is tested further. hopefully it will work correctly when required, but we? like to hear of any problems you do encounter.
7 of 11 elm323 elm323dsa elm electronics ?circuits for the hobbyist < http://www.elmelectronics.com/ > 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. the first byte (known as the ?ode? always describes the type of data being requested, while the second, third, etc. bytes specify the actual information required (given by a ?arameter identification?or pid number). the modes and pids are described in detail in the sae documents j1979 and j2190, and may also be expanded on by the vehicle manufacturers. normally, one is only concerned with the nine diagnostic test modes described in j1979 (although there is provision for more). note that all of these modes are not required to be supported by every vehicle, and are often not. these are the nine modes: 01 : show current data 02 : show freeze frame data 03 : show diagnostic trouble codes 04 : clear trouble codes and stored values 05 : test results, oxygen sensors 06 : test results, non-continuously monitored 07 : test results, continuously monitored 08 : special control mode 09 : request vehicle information within each mode, pid 00 is normally reserved to show which pids are supported by that mode. mode 01, pid 00 must be supported by all vehicles, and can be accessed as follows ensure that the elm323 is properly connected to your vehicle, and powered. most vehicles will not respond without the ignition key in the on position, so turn the ignition on, but do not start the vehicle. at the prompt, issue the mode 01 pid 00 command: >01 00 the first time the bus is accessed, you will see a bus initialization message, followed by the response, which might typically be as follows: 41 00 be 1f b8 10 the 41 00 signifies a response (4) from a mode 1 request from pid 00 (a mode 2, pid 00 request is answered with a 42 00, etc.). the next four bytes (be, 1f, b8, and 10) represent the requested data, in this case a bit pattern showing the pids supported by this mode (1=supported, 0=not). although this information is not very useful for the casual user, it does serve to show that your connection is working. another example requests the current engine coolant temperature (ect). this is pid 05 in mode 01, and can be requested as follows: >01 05 the response will be of the form: 41 05 7b this shows a mode 1 response (41) from pid 05, with value 7b. converting the hexidecimal 7b to decimal, one gets 7 x 16 + 11 = 123. this represents the current temperature in degrees celsius, with the zero value offset to allow operation at subzero temperatures. to convert to the actual coolant temperature, simply subtract 40 from the value. in this case, then, the ect is 123 - 40 = 83 deg c. a final example shows a request for the obd requirements to which this vehicle was designed. this is pid 1c of mode 01, so at the prompt, type: >01 1c a typical response would be: 41 1c 01 the returned value (01) shows that this vehicle conforms to obdii (california arb) standards. the presently defined responses are : 01 : obdii (california arb) 02 : obd (federal epa) 03 : obd and obdii 04 : obd i 05 : not intended to meet any obd requirements 06 : eobd (europe) some modes may provide multi-line responses (09, if supported, can display the vehicle? serial number). the elm323 will attempt to display all responses in these cases, but only if it is allowed sufficient time to process each. there may be occasions when the vehicle responds too quickly and lines are lost. hopefully this has shown how typical requests proceed. it has not been meant to be a definitive source on modes and pids - this information can be obtained from the sae (http://www.sae.org/), from the manufacturer of your vehicle, iso (http://iso.org/), or from various other sources on the web.
interpreting trouble codes 8 of 11 elm323 elm323dsa elm electronics ?circuits for the hobbyist < http://www.elmelectronics.com/ > 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. this is done with a mode 01 pid 01 request as follows: >01 01 to which a typical response might be: 41 01 81 07 65 04 the 41 01 signifies a response to the request, and the first data byte (81) is the result that we are looking for. clearly there would not be 81(hex) or 129(decimal) trouble codes if the vehicle is operational. in fact, this byte does double duty, with the most significant bit being used to indicate that the malfunction indicator lamp (mil, or ?heck engine? has been turned on by one of this module? codes (if there are more than one), while the other 7 bits provide the actual number of stored codes. to determine the number of stored codes, then, one needs to subtract 128 (or 80 hex) from the number if it is greater than 128, and otherwise simply read the number of stored codes directly. the above response then indicates that there is one stored code, and it was the one that set the check engine lamp or mil on. the remaining bytes in the response provide information on the types of tests supported by that particular module (see the sae document j1979 for further information). in this instance, there was only one line to the response, but if there were codes stored in other modules, they each could have provided a line of response. to determine which module is reporting the trouble code, one would have to turn the headers on (ath1) and then look at the third byte of the three byte header for the address of the module that sent the information. having determined the number of codes stored, the next step is to request the actual trouble codes with a mode 03 request: >03 a response to this could be: 43 01 33 00 00 00 00 the ?3?in the above response simply indicates that this is a response to a mode 03 request. the other 6 bytes in the response have to be read in pairs to show the trouble codes (the above would be interpreted as 0133, 0000, and 0000). note that the response has been padded with 00? as required by the sae standard ?the 0000? do not represent actual trouble codes. as was the case when requesting the number of stored codes, the most significant bits of each trouble code also contain additional information. it is easiest to use the following table to interpret the first digit of trouble codes as follows: powertrain codes - sae defined 0 ? ? - manufacturer defined ? ? - sae defined ? ? - jointly defined 1 2 3 if the first hex digit received is this, replace it with these two characters chassis codes - sae defined 4 ? ? - reserved for future 5 6 7 body codes - sae defined 8 9 a b network codes - sae defined c d e f p0 p1 p2 p3 c0 c1 c2 c3 b0 b1 b2 b3 u0 u1 u2 u3 ? ? - reserved for future ? ? - manufacturer defined ? ? - manufacturer defined ? ? - manufacturer defined ? ? - manufacturer defined ? ? - manufacturer defined ? ? - manufacturer defined ? ? - reserved for future taking the example trouble code (0133), the first digit (0) would then be replaced with p0, and the 0133 reported would become p0133 (which is the code for an ?xygen sensor circuit slow response?. as for further examples, if the response had been d016, the code would be interpreted as u1016, while a 1131 would be p1131. had there been codes stored by more than one module, or more than three codes stored in the same module, the above response would have consisted of multiple lines. as stated before, to determine which module is reporting each trouble requires turning the headers on with an ath1 command.
error messages 9 of 11 elm323 elm323dsa elm electronics ?circuits for the hobbyist < http://www.elmelectronics.com/ > resetting trouble codes when hardware or data problems are encountered, the elm323 will respond with one of the following short messages. here is a brief description of each: bus busy the elm323 tried to send the mode command or initialize the bus, but detected too much activity to insert a message. this could be because the bus was in fact busy, but is often due to wiring problems that result in a continuously active input to obdin. fb error this signifies that there was a ?eedback?error. when the k line is first energized, a check is made to ensure that the signal is being fed back to obdin, and if it does not appear there, this message will be provided. check your wiring before proceeding. data error there was a response from the vehicle, but the information was incorrect or could not be recovered. in the case of a bus initialization, this error signifies that the format bytes received were not as expected, so initiation could not take place. if the error occurs during normal operation, it means that the response did not contain enough bytes to be a valid message (which can occur if the signal is interrupted during transmission). example application 10 of 11 the sae j1962 standard dictates that all obd compliant vehicles must provide a standard connector near the driver? seat, the shape and pinout of which is shown in figure 1 below. the circuitry described here will be used to connect to the plug without modification to your vehicle. the male j1962 connector required to mate with a vehicle? connector may be difficult to obtain in some locations, and you could be tempted to improvise by making your own connections to the back of your vehicle? connector. if doing so, we recommend that you do nothing which would compromise the integrity of your vehicle? obd network. the use of any connector which could easily short pins (such as an rj11 type telephone connector) is definitely not recommended. the circuit of figure 2 on the next page shows how the elm323 would typically be used. circuit power has been obtained from the vehicle (via obd pins 16 and 5) and, after some minor filtering, is presented to a five volt regulator. the output of this regulator powers several points in the circuit as well as an led (for visual confirmation that power is present). the remaining two connections to the vehicle (obd pins 7 and 15) are for the two data lines prescribed by the iso 9141 and iso 14230 standards. to meet the standards, the elm323 controls both lines through the npn transistors shown, with the pullup resistors connected to their collectors. the 510 w value for these resistors is specified in the standards, and substituting for a larger value would only increase rise times, likely making the circuit inoperable. reducing the value could cause circuit damage, so try to keep as close as possible to the 510 w . note also that 1/2w resistors should be used (although 1/4w 240 w + 270 w will work, too). data is received from the k line of the obd bus and inverted by the pnp transistor shown before being applied to pin 11 of the elm323. this transistor raises the threshold voltage to about 4v from the inherent 2.5v with the cmos input of the elm323. this helps to increase noise immunity while reducing transition times on the input, due to the amplification. a very basic rs232 interface is shown connected to pins 5 and 6 of the elm323. this circuit ?teals 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). elm323 elm323dsa elm electronics ?circuits for the hobbyist < http://www.elmelectronics.com/ > rs232 data from the computer is directly connected to pin 5 of the ic through only a 47k w current limiting resistor. this resistor allows for voltage swings in excess of the supply levels while preventing damage to the elm323. a single 100k w resistor is also shown in this circuit so that pin 5 is not left floating if the computer is disconnected. transmission of rs232 data is via the pnp transistor shown connected to pin 6. this transistor allows the output voltage to swing between +5v and the negative voltage stored on the 0.1? capacitor (which is charged by the computer? txd line). although it is a simple connection, it is quite effective for this type of application. note also that pin 4 has been tied to v dd , so by default linefeed characters will be sent whenever a carriage return is sent. the four leds shown (on pins 7 to 10) have been provided as a visual means of confirming circuit activity. resistors are shared among tx and rx leds as they will not be on at the same time (the elm323 is not capable of true multitasking). the obd bus may be in an initialization phase while data is being sent or received on the rs232 bus, though, so separate resistors are shown for these two groups. finally, the crystal shown connected between pins 2 and 3 is a common tv type that can be easily and inexpensively obtained. the 27pf crystal loading capacitors shown are typical only, and you may have to select other values depending on what is specified for the crystal you obtain. this completes the description of the circuit. while it is the minimum required to talk to an obd equipped vehicle, this is a fully functional circuit. you may want to expand on it, though, providing more protection from faults and electrostatic discharge, or providing a different interface for the rs232 connection to the computer. then perhaps a basic program to make it easier to talk to the vehicle, and a method to log your findings, and figure 1. vehicle connector 1 9 8 1 6
11 of 11 elm323 elm323dsa elm electronics ?circuits for the hobbyist < http://www.elmelectronics.com/ > figure 2. typical obd to rs232 interface 3 (rxd) 7 (sg) 2 (txd)
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