Method and Apparatus of Applying CRC to Arinc 429 Periodic Data

The present invention relates to periodic error checking of data in an ARINC 429 data bus.

The ARINC 429 data bus is used for transmission of data between avionics units. The bus is a broadcast bus with one transmitter transmitting to multiple receivers. The data path between the sending and receiving units includes many elements such as a transmitter, receiver, input/output controller, memory, and processor interface electronics. An error could be introduced during transmission through any of these elements. If errors are undetected, there could be serious consequences in critical flight avionics applications. This often leads to the incorporation of costly additional redundancy.

A Cyclic Redundancy Check (CRC) exists to check the data transmitted on an ARINC 429 bus and provide the critical means necessary to detect for errors. CRCs are well known in the data communications area of technology. The CRC is performed by defining a dividend based on the data to be checked. The resultant remainder is the CRC. Usually, the CRC is added at the end of the data when it is transmitted. The integrity of the checked data can be verified on the receiving end by reassembling the data, computing the CRC, and comparing it to the received CRC attached to the data. If the CRCs match then no error exists.

Data can be sent over the bus periodically or aperiodically. Aperiodic data is event driven which means that data is requested and in reply to the request, data is sent. Currently, aperiodic data is transmitted in the ARINC 429 as blocks of data which can be thought of as a pulse of data. FIG. 1 shows aperiodic data which has a start, designated by a header, and an end, designated by an end, with data words in between. With aperiodic data, the end of the block is where the CRC information is embedded.

A label is used to give a designation to the data. At the receiving unit, desired data to be checked is identified and sampled by the data label. Sampling is basically retrieving data based on the label desired. Once the data is sampled, the data is retrieved so that a CRC is performed. All aperiodic data in a block has the same label. Therefore, each data word in the block has the same label attached to it. Due to the format of the block, it is important to keep track of the sequence the data is sent so that it may arrive at the receiving unit in the same block order.

Periodic data, on the other hand, is sent periodically with the data words sent in any order and not in the form of a block. The transmission is time critical and clock driven. However, with this type of databus and associated avionics architecture, the clocks of the transmitting and receiving units are not synchronized. As a result, the data could be sampled such that the received CRC check word corresponds to a different data set than the sampled data set. The CRC computed would not match the CRC attached and an error signal would be sent when in fact, an error may not truly exist. Without a synchronization mechanism, this occurs often enough to render CRC checking unsafe.

FIG. 2 shows an example of erroneous sampling which is symbolized by the dashes. As stated before, this erroneous sampling results in errors detected incorrectly.

Each block of five words represents a message and associated CRC. The horizontal line with an arrowhead indicates the direction the data is moving. Therefore, block A moves through the databus first, and then block B and then block C. As can be seen in block B, for example, one word from block A data set is sampled along with an incomplete block B data. This results in an inconsistent sampled data set and consequent erroneous failure of the CRC.

As a result, it would be beneficial and critical to have a means available for CRC integrity checking for periodic data which overcomes the inherent limitations of asynchronous sampling.

A method is disclosed of providing an error free error detection of critical aircraft data transmitted periodically from a sending unit over a data bus to a receiving unit. The error free aspect of the method is provided for by designating a key word at the end of a packet of data to be sent by the sending unit. The packet of data is then sent through the datastream in the databus and the receiving unit receives the packets of data sent. The receiving unit is preprogrammed to sample the data upon receipt of the keyword and the error detection is performed on the sampled packets of data.

FIG. 1 shows how data was previously transmitted in a block form.

FIG. 2 shows how data was sampled previously.

FIG. 3 shows the architecture of the present invention.

FIG. 4 shows data sampling by present invention.

A method is disclosed to provide CRC integrity for ARINC 429 periodic data. FIG. 3 shows the structure required to perform this method. A transmission unit 2 transmits a data message made of a plurality of data words. Transmission units are well known in this area of technology and any ARINC 429 transmission unit which transmits aircraft data can be used. The message is sent to a means for determining the CRC 4. Once the CRC is determined for the message, it is sent to a formatting and transmitting means 6 that adds the CRC to the message. In the method of the present invention, a keyword, designated by a specific label, is the designation of the end of the data message. The data message along with the CRC and the keyword is called a packet of data. The keyword may be transmitted as part of the data message words or the CRC, but must be transmitted last.

The packet of data leaves the transmitting unit 2 and enters the datastream in the ARINC 429 databus 10. The packet of data passes from the datastream to a receiving unit 12.

The data packet enters an input buffer 14 in the receiving unit 12 wherein messages are stored. Receiving units are well known in this area of technology and any receiving unit in this technology can be used with the present invention. The receiving unit 12 is preprogrammed to know which data words are desired, as well as which label designates the keyword.

In accordance with this aspect, one method for the receiving unit 12 to know which data words are desired is to compare or match the keyword provided in the data packet to a keyword preprogrammed at the receiving unit 12. Upon receipt of the keyword in the input buffer 14, the data packet is sent from the input buffer 14 to a packet buffer 18. As a result, the data message to be checked by the CRC is synchronized to the data message for which the CRC was computed. Checking is not dependent on transmission order and all the burdens of tracking are no longer required.

The keyword is what triggers a data message to be sampled and is designated by a switch 16 in FIG. 3. With the keyword last, the data message is received before the keyword is sampled and therefore, no errors will occur in sampling. FIG. 4 shows the keyword sampling of the present invention. Sampling occurs once a keyword is recognized and with the keyword last, the entire data set, whether it is block A, B, or C is sampled completely as opposed to the asynchronous sampling that was previously performed as shown in FIG. 2.

The selected data message is then sent from the packet buffer 18 to an assembling and comparing means 20 to assemble the message to be checked by the CRC. After the reassembling operation occurs, a CRC is computed from the received message and is compared to the CRC attached to the received message. If the CRCs match, a valid signal is given. Otherwise, an error signal is given. As can be seen, the present invention provides the capability to perform a CRC check in an error free manner.

This invention has been described herein in considerable detail in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment details and operating procedures, can be accomplished without departing from the scope of the invention itself.