The controller of interface of the personal computer with onboard system of self-diagnostics OBD II under reports of standards SAE (PWM and VPW) and ISO 9141-2

To standard VPW models of manufacture of company GM answer, to standard PWM - Ford, to standard ISO 9141-2 - the Asian and European models.

The general data

The considered device represents the microcontroller executed on technology КМОП (CMOS).

The controller 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 the data of system OBD II (engine turns, temperature of a cooling liquid and soaked up air, loading characteristics, the expense of air arriving in the engine, etc.) within the limits of standard SAE J1979 through the tyre of any execution (PWM, VPW and ISO 9141-2).

Main destination

To the computer it is enough for connection 3-zhilnogo a wire, connection to a diagnostic socket is carried out 6-zhilnym by a wire. Pressure of a food moves on the controller through 16-contact diagnostic socket OBD. The basic scheme of the controller is presented more low.

The scheme of the controller of interface of the personal computer with onboard system of self-diagnostics OBD II

Recommendations about application

For device connection to the car than 1.2 m that has special value at use of report PWM can be used неэкранированный a cable, length no more. At cable use большей lengths it is necessary to reduce resistance of resistors on a device input (R8 and R9 or R15). At use экранированного a cable, the screen should be disconnected for the purpose of capacity decrease.

The cable for connection to a computer serial port also can be неэкранированным. The device stably works with a cable in length to 9 m. At much большей to length of a cable it is necessary to use more powerful коммуникатор RS 232.

The topology of electric connections is any. At the raised humidity apply additional shunting condensers.

The free software (browser) for reading of codes and the data can be downloaded from sites of manufacturers, or a site of our publishing house, and is intended for use under DOS. The insignificant size of a program application in a variant under DOS allows to contain it on loading diskette DOS and to use even on the computers equipped incompatible with DOS with the software. An unessential condition is even presence in the hard disk computer.

The general principles of data exchange

If the opposite is not stipulated especially, all numbers are resulted in 16-richnom a format (hex).

The decimal format is designated by a label dec.

Data exchange goes on three-wire consecutive connection, without application инициализационного an exchange of office messages (handshaking). The device listens to the channel on presence of messages, executes accepted commands and transfers results to the personal computer (PC) then immediately comes back in a listening mode. Entering into the controller and the data starting with it are organised 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 quantity of information bytes following further. So, for example, 3-bajtnaja the command will look as follows: 03 (control byte), 1st byte, 2nd byte, 3rd byte. The similar format is used as for entering commands on interrogation of onboard system of self-diagnostics, and for the outgoing messages containing the 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 commands and can be used PC at initialization of connection with the controller and the coordination of the report of data transmission, and also the controller for the control of errors of transfer. In particular, in case of an error by transfer, the controller makes installation of the senior meaning bit (MSB) control byte in unit. By successful transfer all four senior bits are established in a zero.

There are separate exceptions to the rules of use of control byte.

Initialization of the controller and onboard system of self-diagnostics

To start data exchange PC should make installation of connection with the controller, then initialize the controller and the channel of data OBD II.

Connection installation

After controller connection to PC and to diagnostic socket OBD its initialization for the purpose of prevention of the "lags" connected with noise in consecutive lines in a case if their connection should be made has been made before inclusion of a food of the controller. The elementary check of activity of the interface is simultaneously made. First of all the one-byte signal 20 (hex), perceived by the controller as a command on connection installation is sent. In the answer the controller instead of the control sends unique byte FF hex (255 dec) and passes in a mode of expectation of reception of the data. Now PC can pass to initialization of the channel of the data.

The case in point is one of the few when the controller does not use control byte.

Initialization

At the given stage report initialization on which data exchange will be made, and in case of report ISO – initialization of onboard system is made. Data exchange is made on one of three reports: VPW (General Motors), PWM (Ford) and ISO 9141-02 (the Asian/European manufacturers).

There is a set of exceptions: so, for example, at interrogation of some models of cars Mazda it can be used фордовский report PWM. Thus, at occurrence of problems of transfer it is necessary first of all to try to take advantage of any other report.

The report choice is made by transfer of the combination consisting of control byte 41 (16-richn.) and the following it is direct behind it of the byte defining type of the report: 0 = VPW, 1 = PWM, 2 = ISO 9141. So, for example, at the command of 41 (hex) 02 (hex) initialization of report ISO 9141 is made.

In the answer the controller sends control byte and condition byte. Installation MSB of control byte speaks about presence of the problems, the byte of a condition thus following it will contain the corresponding information. At successful initialization the control byte 01 (hex), specifying that follows further верификационный condition byte is sent. In case of reports VPW and PWM верификационный the byte represents a simple echo of byte of a choice of the report (0 or 1, accordingly), at initialization of report ISO 9141 it will be the digital key returned by onboard processor OBD and defining which of two versions of the report slightly different from each other will be used.

The digital key has purely information appointment.

It is necessary to notice that initialization of reports VPW and PWM occurs much faster as demands only transfer of the corresponding information to the controller. On the models answering to standard ISO, initialization occupies an order of 5 seconds spent for an exchange of the controller by the information with the onboard processor, made with a speed 5 бод. It is necessary to pay attention of the reader that on some models of cars of family ISO 9141 initialization of the report stops, if the inquiry about delivery of the 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 report initialization the regular data exchange, consisting of arriving from PC inquiries and answers given out by the controller begins.

Data exchange order

Controller functioning at use of reports of family ISO 9141-2 and SAE (VPW and PWM) occurs under a little various scenarios.

Exchange under reports SAE (VPW and PWM)

At data exchange under the given reports there is a buffering only one shot of the data that means necessity of a concrete definition subject to capture or shot return. In some (rare) cases the onboard processor can transfer the packages consisting more than from one shot. In such situation the inquiry should repeat until all shots of a package will not be accepted.

The inquiry always is formed as follows: [Control byte], [Inquiry under standard SAE], [shot Number]. As already it was mentioned above, the control byte usually represents the number equal to full quantity of bytes following it. The inquiry is made out according to Specifications SAE J1950 and J1979 and consists of heading (3 bytes), sequence of information bytes and byte of the control of an error (CRC). We will notice that while the information on demand is formed in strict conformity with Specifications SAE, the consumer of control byte and shot number is the interface chip.

At successful end of procedure the reciprocal message always has the following format: [Control byte], [the Answer under standard SAE]. The control byte, as well as earlier, defines quantity of information bytes following it. The answer according to requirements of standard SAE consists of heading (3 bytes), a chain of information bytes and byte CRC.

At failure it is sent 2-bajtnoe the reciprocal message: [Control byte], [condition Byte]. Thus in control byte installation MSB is made. Four younger bits form number 001, testifying that the control is followed by unique byte, - condition byte. The given situation can arise often enough as Specifications suppose possibility of a lack of distribution the onboard processor of the data, and also transfer of the incorrect data to a case when the inquiry does not correspond supported by manufacturers of the car to the standard. The situation when the required data is absent in operative memory of the processor in time present situation is possible also. When the chip does not receive the expected answer, or obtains the damaged data, installation MSB of control byte is made, and after the control the condition byte stands out.

At collisions in the tyre the interface develops unique byte 40 (hex), being in control byte with the nulled younger bit. The similar situation can arise often enough at loading of an automobile tyre by messages of higher than at the diagnostic data of a priority, - the computer should repeat initial inquiry.

Exchange under reports ISO 9141-2

Standard ISO 9141-2 is used by the majority of the Asian and European manufacturers of automobile technics. The structure formed PC inquiry a little than differs from used in standards SAE with that only a difference that the chip does not require the information on number of a shot and the corresponding 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 the control sum following it. As the reciprocal message the chip simply relays the signals generated by the onboard processor. The control byte in the reciprocal message is absent, therefore PC perceives the arriving information continuously until the chain does not interrupt the pause the in length of 55 milliseconds informing on end of an information package. Thus, the reciprocal message can consist of one or more shots according to requirements of Specifications SAE J1979. The chip does not make the analysis of shots, does not reject not diagnostic shots etc. PC should make own forces processing of the arriving data for the purpose of exarticulation of separate shots by the analysis of heading bytes.

Answers to the majority of inquiries consist of a unique shot.

The updatings made in the interface controllers of last versions

The basic differences of process of data transmission under reports SAE and ISO 9141, characteristic for the interface controllers of last versions, and also a data transmission order under report ISO 14230 are more low resulted:

Standard ISO 9141: the address byte Is added; Standard ISO 9141: return not one, and both key bytes Is carried out; (the additional byte comes back also in modes SAE, however here it is not used). Support of report ISO 14230 is added.

Connection installation

The order of installation of connection has not changed:

Sending:	20
Reception:	FF

Report choice

The typical affirmative reply looks as follows: S1, S2, … …. = 83, F1, 10, С1, Е9, 8F, BD

ISO 14230 (slow initialization): Similarly ISO 9141

The remark and comments

If use of the controller for data transmission only on any one is planned or to two of reports, superfluous components can be excluded (see the scheme above). For example, at the scheme organisation under report VPW (GM) in a wire of connection of the controller to the car three veins of electroconducting (the plug 16, 5 and 2) be required only.

If report PWM is not used, elements R4, R6, R7, R8, R9, R10, Т1, Т2 and D1 can be excluded.

At refusal of an exchange under report ISO elements are subject to an exception: R15, R16, R17, R18, R19, R21, Т4 and Т5.

Refusal of use of report VPW allows to exclude following elements: R13, R14, R23, R24, D2, D3 and Т3.

Ugolno-film resistors with the 5 percent admission of resistance are applied.

Pay attention to absence of the button of emergency reboot (RESET), in case of need such reboot can be made by a detachment of the controller from an automobile socket (reboot of the interface processor will occur automatically). Software restart on PC leads to new initialization of the interface.