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ch10_handbook:data_file_interpretation

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DATA FILE INTERPRETATION

Overall Data File Organization

Chapter 10 data files are organized as a sequential series of data packets. Each data packet can only contain one type of data (i.e. 1553, Pulse Code Modulation (PCM), etc.). Data packets frequently contain multiple individual data messages.

The Chapter 10 standard requires that the first data packet in a data file be an IRIG 106 Chapter 9 format, Telemetry Attributes Transfer Standard (TMATS) packet. The TMATS packet is used to configure the recorder and to describe the data being recorded. Also, TMATS is stored in a Computer Generated Data, Format 1 (Data Type = 0x01) data packet and is discussed in paragraph 6.5.2.

An important field in the packet header for the TMATS packet is the “IRIG 106 Chapter 10 Version” field. The value of this field determines the specific version of the Chapter 10 standard to use when interpreting the rest of the data file. This value should not be confused with the Data Type Version (but sometimes called Header Version) discussed in paragraph 6.3. The IRIG 106 Version field is only defined for the IRIG 106-07 and later versions of the IRIG 106 standard. Since unused reserved fields must be initialized to zero, this field will have a value of 0 in data files compliant with IRIG 106 prior to IRIG 106-07.

Starting with publication of IRIG 106-07, more than one TMATS packet is allowed at the beginning of the file if recorder configuration information exceeds the packet size limit.

It is required that a Time Data packet be the first “dynamic” data packet. Dynamic data packets are not defined in the Chapter 10 standard but it is generally understood that a dynamic data packet means any data packet other than Computer Generated Data Format 1 (setup record). The purpose of this requirement is to allow the association of clock time with the Relative Time Counter (RTC) before encountering the first data packet in the data file. Programmers are cautioned to verify a valid time packet has been received before attempting to interpret the RTC as described in paragraph 6.6.

A root index packet will be the last data packet in the data file if file indexing is used. The presence of data file indexing is indicated in the TMATS setup record.

The size of all data packets is an integer multiple of 4 bytes (32 bits) with the maximum size of 524,288 bytes. Computer Generated Data, Format 1 (TMATS) packets have a maximum packet size of 134,217,728 bytes. To provide this 4 byte alignment, padding bytes are added, if necessary, to the end of a data packet, just before the checksum. Regardless, when defining data structures representing Chapter 10 data packets and messages, these structures should be forced to be byte aligned by using the appropriate compiler directive.

Some data packet elements are two or more bytes in length. For example, the first data element of a data packet is a two byte sync pattern. Multiple byte data elements are stored in little endian format. That is, the least significant portion of the data is stored at the lowest byte offset.

Data packets are written to disk roughly in the time order in which they are received, but data packets and data messages can occur in the data file out of time order. This order can occur because data recorders receive data simultaneously on multiple channels, each channel buffering data for a period of time and then writing it to disk. Therefore, individual data messages will, in general, be somewhat out of time order because of grouping by channel. Consider the case of two 1553 channels recording the same bus at the same time in an identical fashion. Each channel receives, buffers, and writes data to disk. The first channel will write its buffered data to disk followed by the second channel. The data received from the second channel will be from the same time period as the data from the first channel and will have identical time stamps but will be recorded after the first channel in the data file.

Starting with the IRIG 106-05 standard, recorders are only allowed to buffer data for a maximum of 100 milliseconds and data packets must be written to disk within one second. This ensures that data packets can only be out of time order by a maximum of one second. Be warned, though, that the maximum amount of time data packets can be out of order for data files produced before IRIG 106-05 is unbounded and it is not unusual to encounter data files with data packets five or more seconds out of time order.

Example source code that demonstrates basic parsing of Chapter 10 data files can be found in Appendix A. An example program that demonstrates reading and interpreting a Chapter 10 file can be found in Appendix B.

Overall Data Packet Organization

Overall data packet organization is shown in Figure 6-1. Data packets contain a standard header, a data payload containing one or multiple data messages, and a standard trailer. The standard header is composed of a required header, optionally followed by a secondary header. The data payload generally consists of a Channel Specific Data Word (CSDW) record followed by one or more data messages.

ch10_handbook/data_file_interpretation.1397399627.txt.gz · Last modified: 2014/04/13 09:33 by bob

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