The present invention relates to a multiplex transmission system for a vehicle. Particularly, to a system in which a priority code, which is a code for priority control, is changeable in accordance with a traffic condition of the multiplex transmission path.
The greater use of electronics in vehicles has been accompanied by a number of serious problems, among which are the enlargement in size and the complexity of wire harnesses that interconnect the electronic components. Multiplex communication has become a means for solving these problems, particularly in the automotive field. Multiplex communication refers to transmission, by time-division multiplexing, and basically employs serial transmission.
For example, an EGI multiplex control node for an engine control system, a 4WS multiplex control node for a four wheel steering system, and an ABS multiplex control node for an anti-lock braking system are connected to the multiplex transmission path.
In this multiplex transmission system for vehicles, information for the vehicle, for example, operating information and driving information, are transmitted in units of a frame F each having the frame format as shown in FIG. 2. This frame F consists of an SD (Start Delimiter), a priority code, a frame ID code, data length, data 1 to N, and a check code.
First, the "SD code" is a special code indicating the beginning of the frame F. The reception-side multiplex control node recognizes the start of the frame F upon receiving the SD code.
The "priority code" is a code for indicating the priority of the frame F to be transmitted as a transmission on the multiplex transmission path. If a plurality of multiplex control nodes send the respective frames to the multiplex transmission path at the same time, (i.e., collision of signals happens), each multiplex control node judges whether it should keep sending its frame or not, by comparing its priority to another frame's priority. The multiplex control node sending the frame with a lower priority stops sending its frame at the time of judging its order of priority. Meanwhile, the multiplex control node sending its frame with a higher priority keeps sending its frame, so that this frame with a higher priority is transmitted as a transmission on the multiplex transmission path.
The "frame ID code" is a code for indicating the kind of data which is to be assigned to each bit of a data area. In other words, it indicates the control node in which the frame is used.
The "data length" of FIG. 2, is the number of data which are written in that frame. When there are N number/numbers of data, "N" is sent as a data length. A multiplex control node that receives this frame reads out as much data as corresponds to the data length and a field that follows the data is a CRC check code (error detection code). By confirming this code, it can be confirmed that the end of the frame has been reached.
An explanation will now be given of the reception confirmation signal area (ACK field) of FIG. 2. This field is made up of a plurality of bits, for instance, 16 bits. A predefined bit area has been assigned to each of the multiplex control nodes. The control nodes on the transmission side check whether a reception has been made normally on the reception side based on each bit of this ACK field. The procedure for using the ACK field will now be explained in more detail. It is assumed that in the ACK field of FIG. 2, control nodes A, B, and C are assigned to bits 1, 2, and 3, respectively. A certain control node on the transmission side sets the bit of the 16-bit ACK field at a position corresponding to the control node itself to "0" and sets the other bits to "1". That is, the control node transmits one "0" bit and fifteen "1" bits following the transmission frame with a predetermined gap to the transmission path. The multiplex control nodes on the reception side check whether or not the contents of the received frame are correct using a check code. If the multiplex control nodes have found no errors, each of the control nodes transmits the corresponding ACK bit which is set to "0" as a reception confirmation signal. As a result of the above procedure, a control node whose ACK bit is "0" indicates that the control node has properly received the frame.
The control of the transmission of the ACK bit is not directly related to the present invention, but it is explained in detail in U.S. patent application Ser. No. 426,399 (filed on Oct. 25, 1989). U.S. patent application Ser. No. 426,399 is incorporated into the present application by reference.
The priority code can be determined by various policies. For example, in the present technology for an automotive vehicle, response to transmitting vehicle information is employed. The EGI control node receives information from each of the sensors frequently, and transmits corresponding information to the other control nodes immediately. The EGI control node has a high priority. On the other hand, 4WS control node and ABS control node which carry out control related to safety, do not have such a high response priority as is in the case of the EGI control node, since their information is not changed as often. Therefore, in the present technology as mentioned above, higher priority to EGI control node and lower priority to 4WS control node and ABS control node are given by a policy to attach importance to their responses. This priority is fixed and can not be changed.
The multiplex transmission system explained above has the following disadvantage.
When a vehicle runs on a winding road, signals from the 4WS control node are transmitted more frequently than in the case of a straight road. As a result, traffic on the transmission path increases and the transmission of the information from 4WS control node, which has a lower priority than that of the EGI control node, is delayed. Furthermore, when the braking control is operated, a signal from the ABS control node is transmitted more frequently. As a result, traffic on the transmission path increases, and the transmission of the information from ABS control node, which has a lower priority than that of the EGI control node, is delayed.
However, control of the 4WS control node and the ABS control node is important to maintain safety of a vehicle. In this regard, it is required to transmit the information from the 4WS control node and the ABS control node as soon as possible, even when the traffic on the transmission path is crowded. On the other hand, fixing a lower priority for the EGI control node than the 4WS control node, results in a disadvantage of losing responsiveness from the engine control.