An electrical data transmission system

Abstract: An electrical data transmission system comprises a BCM 10 and an ASIC 12, interconnected by a serial link (11) which comprises two lines (J, M). The serial link 11 is operative in one of two modes. The first mode is a synchronous mode wherein one of the lines (J, M) acts as a data line and the other (M, J) acts as a synchronisation line; the second mode is an asynchronous mode wherein one of the lines (J, M) is adapted to carry asynchronous data. The serial link 11 is arranged to automatically reconfigure itself to operate in the asynchronous mode if the synchronous mode breaks down.

Description: The present invention relates to an electrical data transmission system, and particularly but not exclusively to a serial link for an automotive multiplexed wiring system, the link interconnecting two modules of an electronic system for a motor vehicle. It is known from US4694504, WO 90/15493, US4593282 and US5329531 to provide a communications system in which both synchronous and asynchronous communications can share the same communication medium. The communication medium is shared by prioritising the transmissions and allocating time slots for access to the data bus. In all these prior art documents, the communications are either synchronous or asynchronous and always stay in that form. It is a problem with all the above mentioned systems that if there is a breakdown in the synchronous transmissions, it is necessary to re-transmit the data asynchronously. Accordingly the invention provides an electrical data transmission system comprising first and second nodes connected by a serial link which includes at least two lines, wherein the system is operable in a first synchronous operating mode in which one of the lines acts as a data line and another of the lines acts as a synchronising line, and characterised in that the system is also operable in a second asynchronous operating mode in which the second node is adapted to receive data asynchronously from one of the lines, and the link is arranged normally to operate in the synchronous mode, and automatically to be reconfigured to operate in the asynchronous mode on detecting one of the lines as inoperative, the asynchronous data using the other of the lines. The second node may be arranged such that it can receive data from either line in the asynchronous mode. In the synchronous operating mode, signals may be sent between the nodes in either direction. In the asynchronous operating mode, the system may be arranged to transmit signals between the first and second nodes in either direction. The second node may have means operative to distinguish between a line carrying data and an inoperative line. At least one of the lines may be adapted to carry data in encoded form, the data including an embedded clock. The invention may also provide a method of operating a serial link for the transmission of data, the link having at least two lines interconnecting first and second nodes characterised in that the method comprises transmitting data synchronously over the lines in a normal mode of operation with one of the lines acting as a data line and another as a synchronising line and, if one of the lines fails, transmitting data asynchronously over the or at least one of the remaining lines in a failure mode. The method may also provide for data transmission in either direction between the first and second node in the synchronous mode or in the asynchronous mode. The method may include detecting line failure and, in response to such failure detection, automatically reconfiguring the second node to read received data asynchronously. The method may also provide that the data transmitted in the normal mode is the same as the data transmitted in the failure mode. The method may include, in the failure mode, reading the signal level of the transmitted data at a predetermined delay after a signal level transition in a predetermined direction. The invention will now be described by way of example with reference to the accompanying drawings, in which: figure 1 is a schematic diagram of a part of a multiplexed wiring system including a serial link according to the invention; figure 2 is a timing diagram of signals carrying coded data on the serial link of figure 1; figure 3 is a fragmentary timing diagram of the signals of figure 2 carrying coded "1" and "0" bits; and figure 4 is a flow chart showing the selection of data reception path according to the invention. With reference to figure 1, part of a vehicle multiplexed wiring system comprises a first node performing the function of a master in the form of a body control module (BCM) 10, bi-directionally connected by a serial link 11 to a second node performing the function of a slave in the form of an application specific integrated circuit (ASIC) 12. The serial link 11 comprises two lines J, M which are made of insulated copper wire. The BCM 10 and ASIC 12 are further connected to a common supply V and a common ground GND. The ASIC 12 includes first, second and third shift registers A, B, C, each of which has a data input A1, B1, C1 and a clock input CKA, CKB, CKC respectively. The signals input through the data inputs A1, B1, C1 are derived by the ASIC 12 from the signals on the J and M lines. Referring now also to figures 2 and 3, in normal operation data is transmitted serially on both lines using a "return to zero" (RZ) coding in which the line stays (or "idles") high until a signal is transmitted, when each bit begins with a low portion and ends with a high portion and has a centre portion which is high for a "1" and low for a "0". It can be seen from figure 2 that the data carried on the J line is binary 101101. The signal on the M line is identical to the signal on the J line and carries the same data at the same data rate but is transmitted with a 180° phase lag which equates to a delay of δ after the signal on the J line, where δ=t/2 and t is the duration between successive falling edges of either line. The 1/2 bit delay δ is also used below to define the embedded clock functions used for the register clocks CKA, CKB, CKC. The ASIC 10 derives the data inputs A1, B1, C1 and the clock inputs CKA, CKB, CKC from the signals on the J and M lines. As the J and M lines "idle high", the registers A, B, C detect the start of a data byte by the first transition from high to low on the line J, M, in question. The byte is then read in using their respective clocks CKA, CKB, CKC. In register A, the signal on the J line is applied directly to the data input A1 to register A, and the signal on the M line is applied directly to the clock input CKA. As can be seen in Figure 2, each falling edge in the signal on the M line coincides with the centre portion of the corresponding bit in the signal on the J line. Therefore, on each falling edge in the signal on the M line, the level of the signal on the J line is stored as a bit of data DA in the register, as a "1" if it is high, and a "0" if it is low. The signal on the M line therefore acts as a clock to time the input of data from the J line into the register. In register B, the signal on the J line is applied directly to the data input B1. The signal on the J line is also applied to the clock input CKB, but after a time delay of δ from when it is applied to the data input. The signal on the J line can therefore be input as data DB to the register B in a similar way to the input to register A, but using the falling edges between the bits on the J line as an embedded clock to time the input of data from the data input. The signal on the J line therefore becomes the data DB held in register B. In register C, the signal on the M line is applied directly to the data input C1, and is applied to the clock input CKC after a time delay of δ from when it is applied to the data input. The data in the signal on the M line can therefore be entered as data DC in the register C in the same way as data is input to register B. The validity of the data DA, DB, DC in each register A, B, C is checked using a checksum. In figure 4, the decision making process for data selection between the shift registers A, B, C is shown and the ASIC 12 automatically configures itself to read the data present in the highest priority shift register A, B, C which holds valid data DA, DB, DC. If data DA is valid, operation of the serial link 11 is in synchronous mode and information is passed through the first shift register A with a flexible bit rate. The data in the other registers B, C is ignored. This is a first operating mode (mode 1). If data DA is invalid, the ASIC 12 will automatically reconfigure itself to operate in a second operating mode (mode 2). If both other data DB, DC are valid, operation of the serial link 11 defaults to a two wire redundant embedded clock with a substantially fixed bit rate. The data is passed using either the J line or the M line and if the line J, M being used develops a fault, the slave will automatically reconfigure itself to read only the other line J, M. The clock CKB, CKC used is the embedded clock for the line J, M being used and the other line J, M with its clock CKB, CKC is redundant. The ASIC 12 gives the second register B a higher priority than the third register C and, as the communications are asynchronous, the bit rate must be kept substantially fixed. In this case the data in register C is ignored. If data DA and data DB are both invalid and data DC is valid, operation of the serial link 11 defaults to a single wire system with an embedded clock and communications are passed asynchronously through the third shift register C but again with a substantially fixed bit rate. If all the data DA, DB, DC is invalid, then a total fault condition exists and communications cannot be established. The BCM 10 will log a fault condition and signal the fault to the user. For ASIC 12 to BCM 10 communications the transmission varies with the operating mode and establishes true bidirectional data transfer. In both cases, the master transmits a header byte followed by a series of "1"s and the slave responds by modifying some of the "1"s to "0"s to produce data which is received by the master. In mode 1, which is synchronous, the timing of the ASIC 12 transmission of data on the J line is controlled by the falling edges in the signal on the M line. In mode 2, which is asynchronous, the timing of the transmission of data by the ASIC, on whichever line is being used, is controlled by the falling edges between the bits in the signal transmitted by the BCM 10 on that line. It will be appreciated that, as with BCM 10 to ASIC 12 transmissions, mode 1 requires both the J and M lines whereas mode 2 may operate on either the J or M line. All the options can be implemented in the ASIC 12 with no requirement for accurate timing or a UART and with common master hardware. This means that a versatile serial link 11 may be provided which can provide a particular mode of operation depending on the requirements of the application thereof. In this way it can be seen that data transmission can be more reliably maintained by the present invention since although synchronous transmission is advantageous for example due to desirable noise thresholds, it is an unreliable mode which breaks down on failure of any one of a plurality of lines, whereas the present invention provides automatic reconfiguration to an asynchronous mode in the event of a breakdown in serial communications.

Claims: An electrical data transmission system comprising first and second nodes (10, 12) connected by a serial link (11) which includes at least two lines (J ,M), wherein the system is operable in a first synchronous operating mode in which one of the lines acts as a data line and .another of the lines (J, M) acts as a synchronising line, and the system is also operable in a second asynchronous operating mode in which the second node (12) is adapted to receive data asynchronously from one of the lines (J, M), and the link (11) is arranged normally to operate in the synchronous mode, and automatically to be reconfigured to operate in the asynchronous mode on detecting one of the lines (J, M) as inoperative, the asynchronous data using the other of the lines (J, M). An electrical system according to claim 1 further characterised in that the second node (12) is arranged such that it can receive data from either line (J, M) in the asynchronous mode. An electrical system according to claim 1 or claim 2 further characterised in that in the synchronous operating mode, signals can be sent between the nodes (10, 12) in either direction. An electrical system according to any one of claims 1 to 3 further characterised in that in the asynchronous operating mode, the system is arranged to transmit signals between the first and second nodes (10, 12) in either direction. An electrical system according to any foregoing claim further characterised in that the second node (12) has means operative to distinguish between a line (J, M) carrying data and an inoperative line. An electrical system according to any preceding claim further characterised in that at least one of the lines (J, M) is adapted to carry data in encoded form, the data including an embedded clock. A method of operating a serial link (11) for the transmission of data, the link (11) having at least two lines (J, M) interconnecting first and second nodes (10, 12) the method comprises transmitting data synchronously over the lines (J, M) in a normal mode of operation with one of the lines (J, M) acting as a data line and another as a synchronising line and, if one of the lines (J, M) fails, transmitting data asynchronously over the or at least one of the remaining lines (J, M) in a failure mode. A method according to claim 7 further characterised in that data is transmitted in either direction between the first and second node (10, 12) in the synchronous mode. A method according to claim 7 or claim 8 further characterised in that data is transmitted in either direction between the first and second node (10, 12) in the asynchronous mode. A method according to any one of claims 7 to 9 further characterised in that the method includes detecting line (J, M) failure and, in response to such failure detection, automatically reconfiguring the second node (12) to read received data asynchronously. A method according to any one of claims 7 to 10, further characterised in that the data transmitted in the normal mode is the same as the data transmitted in the failure mode. A method according to any one of claims 7 to 11 further characterised in that the method includes in the failure mode, reading the signal level of the transmitted data at a predetermined delay δ after a signal level transition in a predetermined direction.

Inventors: GREENWOOD JEREMY JOHN, GREENWOOD, JEREMY JOHN
Applicants: LAND ROVER GROUP LTD, LAND ROVER GROUP LIMITED
Application year: 1996
Main classification: H04L12
IPC codes: H04L12, H04L29