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Self-diagnostics of systems of electronic control of the second generation of OBD II
The controler of interface of the personal computer with onboard system of self-diagnostics of OBD II according to protocols of the SAE standards (PWM and VPW) and ISO 9141-2
Oscillograph application for supervision of signals in chains of control systems
Digital tire of data of CAN
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The controler of interface of the personal computer with onboard system of self-diagnostics of OBD II according to protocols of the SAE standards (PWM and VPW) and ISO 9141-2
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To the VPW standard models of production of the GM company answer, to the PWM standard - to Ford, the ISO 9141-2 standard - Asian and European models. |
General data
The considered device represents the microcontroller executed on KMOP technologies (CMOS).
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The controler is not intended connections to onboard systems of self-diagnostics of the first generation (OBD I)! |
The device carries out a role of the elementary scanner and is intended for reading of diagnostic codes and OBD II system data (engine turns, temperature of cooling liquid and soaked-up air, loading characteristics, a consumption of air arriving in the engine, etc.) within the SAE J1979 standard via the tire of any execution (PWM, VPW and ISO 9141-2).
Main destination
For connection to the computer it is enough 3-жильного a wire, connection to the diagnostic socket is carried out 6-жильным by a wire. The supply voltage moves on the controler via the 16-contact diagnostic OBD socket. The schematic diagram of the controler is presented on an illustration.
Recommendations about application
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PERFORMANCE ORDER |
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General principles of data exchange
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If the opposite is not stipulated especially, all numbers are given in a 16-richny format (hex). |
The decimal format is designated by well-aimed dec.
Data exchange goes on three-wire consecutive connection, without application of an initsializatsionny exchange by office messages (handshaking). The device listens to the channel on existence of messages, executes accepted teams and transfers results to the personal computer (PC) then immediately comes back to a listening mode. Entering into the controler and data starting with it are organized in the form of a chain of bytes consistently going one after another, first of which is control. Usually control byte represents number from 0 to 15 dec (or 0-F hex), describing number of information bytes following further. So, for example, the 3-byte team will look as follows: 03 (control byte), 1st byte, 2nd byte, 3rd byte. The similar format is used as for entering commands for poll of onboard system of self-diagnostics, and for the outgoing messages containing requested information. It is necessary to notice that in control byte four younger bits are used only, the-senior bits are reserved under some special teams and PC can be used at initialization of connection with the controler and coordination of the protocol of data transmission, and also the controler for control of errors of transfer. In particular, in case of a mistake by transfer, the controler makes installation of the senior meaning bit (MSB) of control byte in unit. By successful transfer all four senior bits are established in a zero.
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There are separate exceptions to the rules of use of control byte. |
Initialization of the controler and onboard system of self-diagnostics
To start data exchange of PC should make installation of connection with the controler, then initialize the controler and the channel of data of OBD II.
Connection installation
After controler connection to PC and the diagnostic OBD socket its initialization for the purpose of prevention of the "lags" connected with noise in consecutive lines in case their connection should be made was made before inclusion of a food of the controler. The simplest check of activity of the interface is at the same time made. First of all the one-byte signal of 20 (hex) perceived by the controler as command for installation of connection is sent. In reply the controler instead of the control sends the unique byte of FF hex (255 dec) and passes to a waiting mode of reception of data. Now PC can pass to initialization of the channel of data.
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This case is one of the few when the controler does not use control byte. |
Initialization
At this stage protocol initialization on which data exchange will be made, and in case of the ISO protocol – initialization of onboard system is made. Data exchange is made on one of three protocols: VPW (General Motors), PWM (Ford) and ISO 9141-02 (the Asian/European producers). Remark: There is a set of exceptions: so, for example, at poll of some models of Mazda cars the fordovsky PWM protocol can be used. Thus, at emergence of problems of transfer it is necessary first of all to try to use any other protocol. The choice of the protocol is made by transfer of the combination consisting of control byte 41 (16-richn.) and the byte following directly it defining type of the protocol: 0= VPW, 1 = PWM, 2 = ISO 9141. So, for example, at the command of 41 (hex) 02 (hex) ISO 9141 protocol initialization is made.
In reply the controler sends control byte and condition byte. The MSB installation of control byte speaks about existence of the problems, the byte of a condition thus following it will contain the relevant information. At successful initialization the control byte of 01 (hex) specifying is sent that the verifikatsionny byte of a condition further follows. In case of the VPW and PWM protocols the verifikatsionny byte represents a simple echo of byte of a choice of the protocol (0 or 1, respectively), at ISO 9141 protocol initialization it will be the digital key returned by the onboard OBD processor and defining which of two versions of the protocol slightly differing from each other will be used.
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The digital key has purely information appointment. |
It is necessary to notice that initialization of the VPW and PWM protocols occurs much quicker as demands only transfer of the relevant information to the controler. On the models answering to the ISO standard, initialization takes about 5 seconds, spent for a controler exchange of information with the onboard processor, the 5th baud made with a speed. It is necessary to pay attention of the reader that on some models of cars of ISO family 9141 initialization of the protocol stop, if the request for issue of data is not transferred during a 5-second interval, - told means that PC should make automatic delivery of inquiries everyone some seconds, even in a single mode.
After installation of connection and initialization of the protocol the regular data exchange, consisting of inquiries arriving from PC and answers issued by the controler begins.
Data exchange order
Controler functioning when using protocols of ISO 9141-2 and SAE family (VPW and PWM) occurs according to a little various scenarios.
Exchange according to the SAE protocols (to VPW and PWM)
At data exchange according to these protocols there is a buffering only one shot of data that means need of a specification subject to capture or shot return. In some (rare) cases the onboard processor can transfer the packages consisting more than of one shot. In such situation the inquiry should repeat until all shots of a package will not be accepted.
The inquiry is always formed as follows: [Control byte], [Inquiry on the SAE standard], [Shot number]. As it was already mentioned above, the control byte usually represents the number equal to full number of the following bytes behind it. The inquiry is made out according to the SAE J1950 and J1979 Specifications and consists of heading (3 bytes), sequence of information bytes and byte of control of a mistake (CRC). Let's notice that while information on inquiry is formed in strict compliance with the SAE Specifications, the consumer of control byte and number of a shot is the interface chip.
At a successful conclusion of procedure the response message always has the following format: [Control byte], [The answer on the SAE standard]. The control byte, as well as earlier, defines number of information bytes following it. The answer according to requirements of the SAE standard consists of heading (3 bytes), a chain of information bytes and CRC byte.
At failure the 2-byte response message is sent: [Control byte], [Condition byte]. Thus in control byte the MSB installation is made. Four younger bits form number 001, testifying that the control is followed by the unique byte, - condition byte. This situation can arise rather often as Specifications allow possibility of a lack of distribution the onboard processor of data, and also transfer of incorrect data in a case when the inquiry does not correspond supported by producers of the car to a standard. The situation when required data are absent in random access memory of the processor at the moment time is possible also. When the chip does not receive the expected answer, or obtains the damaged data, the MSB installation of control byte is made, and after the control the condition byte stands out.
At collisions in the tire the interface develops the unique byte of 40 (hex) being control byte with the nulled younger bit. The similar situation can arise rather often when loading a car tire by messages of higher than at diagnostic data of a priority, - the computer should repeat initial inquiry.
Exchange according to the ISO 9141-2 protocols
The ISO 9141-2 standard is used by the majority of Asian and European producers of automobile equipment. The structure of formed PC of inquiry a little than differs from used in the SAE standards with that only a difference that the chip does not need information on number of a shot and the relevant information to be present at a package should not. Thus, the inquiry always consists of control byte and a chain of the information bytes including checksum following it. As the response message the chip simply relays the signals created by the onboard processor. The control byte in the response message is absent, therefore PC perceives arriving information continuously until the chain does not interrupt the pause the in length of 55 milliseconds reporting about end of information package. Thus, the response message can consist of one or more shots according to requirements of the SAE J1979 Specifications. The chip does not make the analysis of shots, does not reject not diagnostic shots etc. PC should make own forces processing of arriving data with the purpose of exarticulation of separate shots by the analysis of heading bytes.
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Answers to the majority of inquiries consist of the unique shot. |
The updatings made in the interface controlers of the last versions
Below the main differences of process of data transmission according to the SAE and ISO 9141 protocols, characteristic for the interface controlers of the last versions, and also a data transmission order according to the ISO 14230 protocol are given:
1) ISO 9141 standard: The address byte is added;
2) ISO 9141 standard: Return not one, and both key bytes is carried out; (the additional byte comes back also in the SAE modes, however here it is not used).
3) ISO 14230 protocol support is added.
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All information bytes are transferred in a 16-richny format. |
Connection installation
The order of installation of connection did not change:
Sending: 20
Reception: FF
Protocol choice
VPW:
Sending: 41, 00
Reception: 02, 01, XX
PWM:
Sending: 41, 01
Reception: 02, 01, XX
ISO 9141:
Sending: 42, 02, adr, where: adr - address byte (usually 33 hex)
Reception: 02, К1, К2, where К1,
К2 - key bytes of ISO
Or: 82, XX, XX (error of initialization of ISO 9141)
ISO 14230 (fast initialization):
Sending: 46, 03, R1, R2, R3, R4, R5, where: R1 ÷ R5 - the message on the beginning of inquiry of ISO 14230 on connection installation, usually R1 ÷ R5 = С1, 33, F1, 81, 66
Reception: S1, S2, … … … - messages on the beginning of the answer of ISO 14230 on connection installation
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Can consistently be transferred more than one ECU. As the answer the negative code of the answer can be used. |
The typical affirmative answer looks as follows:
S1, S2, … …. = 83, F1, 10, С1, Е9, 8F, BD
ISO 14230 (slow initialization):
ISO 9141 is similar
Remark and comments
If controler use for data transmission only on any to one is planned or to two of protocols, superfluous components can be excluded (see the scheme). For example, at the scheme organization under the VPW (GM) protocol in a wire of connection of the controler to the car three veins of electroconducting (plug 16, 5 and 2) be required only.
If the PWM protocol is not used, the elements R4, R6, R7, R8, R9, R10, Т1, Т2 and D1 can be excluded.
At refusal of an exchange according to the ISO protocol elements are subject to an exception: R15, R16, R17, R18, R19, R21, Т4 and Т5.
Refusal of use of the VPW protocol allows to exclude the following elements: R13, R14, R23, R24, D2, D3 and T3.
Coal and film resistors with the 5 percent admission of resistance are applied.
Pay attention to lack of the button of emergency reset (RESET), in case of need such reset can be made by a way of a detachment of the controler from the automobile socket (reset of the interface processor will occur automatically). Software restart on PC leads to new initialization of the interface.
