1 Introduction
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In the process of developing automotive instrumentation testing equipment, a very important task is to realize the serial communication between the industrial computer and the automobile instrument. Through serial communication, it is possible to realize the detection items and parameter setting of a series of instruments before the factory leaving the instrument software and hardware version detection, instrument mileage clearing and instrument calibration. Corresponding to the "New Jetta" type automotive instrument cluster (Siemens VDO company products), in order to achieve technical security, its serial port uses a non-standard serial communication protocol, which is difficult to achieve with general serial communication control, must be Those familiar with the instrument software structure write communication programs from the ground up.
This paper adopts the C++Builder development platform. Under the Windows 98 operating system, it successfully realizes the communication with the “New Jetta†type automobile combination meter, and has a beautiful and simple user interface.
2 NewJetta type instrument serial communication requirements brief
2.1 Hardware connection
The hardware connection between the industrial computer and the instrument in the text is from the K-line of the instrument through the level converter to the RS-232 serial port of the industrial computer, using half-duplex serial asynchronous communication. As shown in Figure 1.
2.2 Software Specification
The software specification includes communication handshake, multiple check, error retransmission, and timeout processing in the serial data communication protocol.
2.2.1 Communication initialization
The communication response can be established when the instrument ignition signal is activated for about 2 s. The information is first transmitted at a 5 b communication rate, which contains 1 start bit, 7 data bits, 1 parity bit and 1 stop bit (**H is sent to the meter).
After receiving the message, the meter waits for approximately 5 ms of cycle time and then the communication module starts. From this point on, all communication bytes will be sent and received at a baud rate of 10 400 b/s (this baud rate is a non-standard baud rate).
After the 100 ms meter sends out the sync byte (1 start bit, 8 data bits, 1 stop bit, **H), the low byte of the password is sent after 10 ms, and the meter sends the password after 10 ms. High byte (1 start bit, 7 data bits, 1 parity bit, 1 stop bit, password value is ****H).
Thereafter, the meter waits for the industrial computer to return the reversed password high byte. This loopback method is the error-proof processing mechanism of the serial communication. If this byte meter is not received, the meter will send the sync byte and password byte again.
When the meter receives the loopback byte, the standard module communication starts at a baud rate of 10 400 b/s (each byte contains 1 start bit, 8 data bits, 1 stop bit). At this point, the meter starts to send the meter ID. During the transmission of these four modules, there are many information response requirements between the meter and the industrial computer. If there is an error, the communication is aborted.
2.2.2 Normal communication process
After the communication is initialized, the normal communication starts with the baud rate of 10 400 b/s. At this time, the industrial computer can send the command byte stream to the instrument to realize the control and parameter setting of the instrument. At regular intervals, the handshake signal must still be sent between the instrument and the industrial computer, otherwise the communication will be aborted.
3 software programming
This communication mode belongs to zero Modem serial asynchronous communication. In asynchronous communication, the character format and baud rate are two important indicators. In this paper, the communication process has both the baud rate change and the frame format change, and the response time of the communication is also strictly required. In the C++Builder visual programming format, functions that directly operate on registers are no longer supported, so the Inporb and Outporb functions are first implemented using embedded assembly. The data flow of this communication is in bytes, so write the byte transceiver function, which is the query mode, such as
4 Conclusion
This paper successfully developed the special serial port communication program of the instrument using C++Builder software platform. The system has been put into use in the production line of Siemens VDO (Wuhu) company for more than one year. Compared with the fast communication method, the communication transmission performance is stable and has The convenient and beautiful user interface lays a foundation for realizing instrumentation testing equipment and has important application value.
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