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PULSE CODE MODULATION STANDARDS
- CHAPTER
- 4.1 General 4- TABLE OF CONTENTS
- 4.2 Class Distinctions and Bit-Oriented Characteristics 4-
- 4.3 Fixed Formats.................................................................................................... 4-
- 4.4 Format Change (Class II) 4-
- 4.5 Asynchronous Embedded Format (Class II) 4-
- 4.6 Tagged Data Format (Class II) 4-
- 4.7 Time Words 4-
- 4.8 Asynchronous Data Merge (Class II) 4-
- Figure 4-1. PCM code definitions. 4- LIST OF FIGURES
- Figure 4-2. PCM frame structure. 4-
- Figure 4-3. 16 bit standardized time word format................................................................ 4-
- Figure 4-4. Time word insertion into 12 bit PCM word size. 4-
- Figure 4-5. Asynchronous word structure.......................................................................... 4-
- Figure 4-6 Overhead truth table. 4-
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The use of fixed frame formats has been a common practice but does not fit all requirements. A verification of range capabilities should be made prior to incorporation of class II features into a telemetry system.
4.2.2 Bit-Oriented Definitions and Requirements. Definitions and requirements relating to serial PCM bit streams are described next.
4.2.2.1 Binary Bit Representation. The following code conventions for representing serial binary ones and zeros are the only permissible representations. Graphic and written descriptions of these conventions are shown in Figure 4-1. Only one convention shall be used within a single PCM bit stream. If Randomized NRZ-L (RNRZ-L) is transmitted, it shall use the 15-bit regeneration pattern as described in Appendix D.
- NRZ-L Biφ-L
- NRZ-M Biφ-M
- NRZ-S Biφ-S
4.2.2.2 Serial Bit Stream Transitions. The transmitted or recorded bit stream shall be continuous and shall contain sufficient transitions to ensure bit acquisition and continued bit synchronization, taking into account the binary representation chosen. (See recommendation in paragraph 1.3, Appendix C.)
4.2.2.3 Bit Rate. The RF and recording limits, defined in Chapters 2 and 6, should be considered when determining maximum bit rates. The minimum bit rate shall be 10 bps. Bit rates greater than 10 Mbps are class II.
4.2.2.4 Bit Rate Accuracy and Stability. During any period of desired data, the bit rate shall not differ from the specified nominal bit rate by more than 0.1 percent of the nominal rate.
4.2.2.5 Bit Jitter. The bit jitter shall not exceed ±0.l of a bit interval referenced to the expected transition time with no jitter. The expected transition time shall be based on the measured average bit period as determined during the immediately preceding 1000 bits.
4.3 Fixed Formats
Characteristics of fixed formats are described below. Fixed formats do not have changes during transmission with regard to frame structure, word length or location, commutation sequence, sample interval, or measurement list.
4.3.1 Word-Oriented Definitions and Requirements. The following definitions and requirements are addressed to word characteristics.
4.3.1.1 Word Length (Class I and II). Individual words may vary in length from 4 bits to not more than 32 bits in class I and not more than 64 bits in class II.
Telemetry Standard RCC Document 106-07, Chapter 4, September 2007
Code
Logic Waveform
Levels
Code Waveforms
Code Definitions
Non Return to Zero - Level
NRZ-L
n
“ONE” is represented by one level
Figure 4-1.
PCM code definitions.
m
“ZERO” is represented by the other level Non Return to Zero - Mark
NRZ-M
n
“ONE” is represented by a change in level m
“ZERO” is represented by
NO
change in level
Non Return to Zero - Space
NRZ-S
n
“ONE” is represented by
NO
change in level
m
“ZERO” is represented by a change in level Bi-Phase - Level
(1)
Bi
φ-L
n
“ONE” is represented by a “ONE” level with transition to the “ZERO” level
m
“ZERO” is represented by a “ZERO” level with transition to the “ONE” level Bi-Phase - Mark
(1)
Bi
φ-M
n
“ONE” is represented by
NO
level change at the beginning
of the bit period
m
“ZERO” is represented by a level change at the beginning of the bit period Bi-Phase - Space
(1)
Bi
φ-S
n
“ONE” is represented by a level change at the beginning of the bit period
m
“ZERO” is represented by a
NO
level change at the
beginning of the bit period
Notes:
(1) The Bi
φ^
codes may be derived from the corresponding NRZ codes by inverting the level for the last half of each bit interval.
(^101010101010)
maximum value. In formats where subcommutation is present, the subframe ID counter may serve as the frame counter.
MINOR FRAME SYNC 1 2 (W,1) N-
MAJOR FRAME
(W,S)
MINOR FRAME SYNC 1 2 (W,Z) N-
MINOR FRAME LENGTH “N” WORDS OR “B” BITS
MAX LENGTH
CLASS I - 8 192 BITS OR 1024 WORDS
CLASS II - 16 384 BITS
- BY DEFINITION A MAJOR FRAME CONTAINS NZ WORDS OR BZ BITS “Z” = THE NUMBER OF WORDS IN LONGEST SUBFRAME (MAX. 256). “N” = THE NUMBER OF WORDS IN MINOR FRAME. “B” = THE NUMBER OF BITS IN MINOR FRAME.
- MINOR FRAME SYNC IS CONSIDERED ONE WORD, REGARDLESS OF LENGTH.
- “W” IS WORD POSITION IN THE MINOR FRAME.
- “S” IS WORD POSITION IN THE SUBFRAME.
SUBFRAME OF “Z” WORDS
Figure 4-2. PCM frame structure.
4.3.2.2 Major Frame. A major frame contains the number of minor frames required to include one sample of every parameter in the format.
4.3.2.2.1 Major Frame Length. Major frame length is defined as minor frame length (N words or B bits) multiplied by the number of minor frames (Z) in the major frame. The maximum number of minor frames per major frame shall not exceed 256.
4.3.2.2.2 Minor Frame Numbering. To provide consistent notation, the first minor frame in a major frame shall be numbered "one." Each subsequent minor frame shall be numbered sequentially within the major frame.
4.3.2.3 Subcommutation. Subcommutation is defined as a sampling of parameters at submultiple rates (1/D) of the minor frame rate where the depth of a subframe, D, is an integer in the range of 2 to Z.
4.3.2.3.1 Subframe. Subframe is defined as one cycle of the parameters from a subcommutated minor frame word position. The depth, D, of a subframe is the number of minor frames in one cycle before repetition.
4.3.2.3.2 Subframe Synchronization Method. The standard method for subframe synchronization is to use a "subframe ID counter," a binary counter which counts sequentially up or down at the minor frame rate. Typically, only one subframe ID counter is used in a PCM format; however, more than one counter may be used if needed. This paragraph assumes the use of one subframe ID counter. The subframe ID counter shall be located in a fixed position in each and every minor frame. The counter should start with the minimum counter value when counting up or the maximum counter value when counting down. The counter should also be left or right justified in a word position. The start of a major frame shall coincide with the initial count for the deepest subframe.
4.3.2.4 Supercommutation. Supercommutation ("supercom") is defined as time-division- multiplex sampling at a rate that is a multiple of the minor frame rate. Supercommutation (on a minor frame) provides multiple samples of the same parameter in each minor frame. "Supercom on a subframe" is defined as time-division-multiplex sampling at a rate that is a multiple of the subframe rate and provides multiple samples of the same parameter within a subframe. For class I, supercommutated samples shall be evenly spaced. For class II, supercommutated samples should be as evenly spaced as practical.
4.4 Format Change (Class II)
Format change is defined as change with regard to frame structure, word length or location, commutation sequence, sample interval, or change in measurement list. Format changes shall occur only on minor frame boundaries. Bit synchronization shall be maintained and fill bits used instead of intentional dead periods. Format changes are inherently disruptive to test data processing; fixed format methods are preferred. Format change methods shall conform to the characteristics described in the following sections.
telemetered ARINC 429 data shall be consistent with the Aeronautical Radio Inc. specification of ARINC 429 bus data.
4.7 Time Words
The following paragraphs describe the formatting of time words within a PCM stream. A 16-bit standardized time word format and a method to insert time words into PCM word sizes other than 16-bits are described.
4.7.1 In 16-bit standardized time word format, there shall be three words dedicated to providing timing information. These words are designated high order time, low order time, and microsecond time. High and low order time words shall be binary or binary coded decimal (BCD) weighted, and microsecond words shall be binary weighted. Time word construction examples are shown in Figure 4-3 and Figure 4-4.
4.7.2 The microsecond time word shall have a resolution of 1 microsecond; that is, the least significant bit, bit 16, has a value of 0.000001 second. This word shall increment until it attains a value of 10 milliseconds at which time it will reset to zero. Thus the maximum value of the counter is 9999 (decimal).
4.7.3 The low order time word shall have a resolution of 10 milliseconds; that is, the least significant bit, bit 16, of the low order time word shall have a value of 0.01 second.
4.7.4 The high order time word shall have a resolution of 655.36 seconds when binary weighted; that is, the least significant bit, bit 16, has a value of 655.36 seconds. When BCD weighted, the least significant bit, bit 16, of the high order time word shall have a value of one minute. For BCD, the days field shall contain the three least significant characters of the BCD Julian date.
4.7.5 It is recommended that high, low, and microsecond time words precede the first data word in the minor frame. The time word order shall be high order time word, followed by low order time word, followed by microsecond time word. Microsecond time words may be used to tag individual data words, but care shall be taken that high order and low order time words be inserted at a rate necessary to resolve time ambiguities.
4.7.6 Time word insertion into PCM word sizes other than 16 bits shall be as follows: high order, low order, and microsecond time words shall be inserted into PCM words with time word bits occupying contiguous bit locations in the PCM word. The time word shall occupy contiguous PCM data words until the time word is contained in the PCM stream. If the time word size is not an integer multiple of the PCM word size and there are unused bits in the PCM word, the remaining unused bits in the last PCM word that contains the time word shall be fill bits with value 0. Figure 4-4 illustrates the insertion of time words into a PCM stream with word size of 12 bits.
HIGH ORDER TIME 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1 DAY 10 HR 1 HR 10 MIN 1 MIN 655.36 SEC LOW ORDER TIME
(BCD Weighting) (Binary Weighting)
10 SEC 1 SEC 0.1 SEC 0.01 SEC 0.01 SEC MICROSECOND TIME
(BCD Weighting) (Binary Weighting)
(Binary Weighting) 1 MICROSECOND
Figure 4-3. 16 bit standardized time word format.
HIGH ORDER TIME
PCM WORD N PCM WORD N+ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 24 ALL ZERO FILLER
1 DAY 10 HR 1 HR 10 MIN 1 MIN 655.36 SEC
LOW ORDER TIME
PCM WORD N+2 PCM WORD N+
(BCD) (Binary)
0 ALL ZERO FILLER
10 SEC 1 SEC 0.1 SEC 0.01 SEC 0.01 SEC MICROSECOND TIME
PCM WORD N+4 PCM WORD N+
(BCD) (Binary)
0 0 ALL ZERO FILLER
(Binary Weighting) 1 MICROSECOND
Figure 4-4. Time word insertion into 12 bit PCM word size.
Figure 4-5. Asynchronous word structure.
(MSB) (LSB) B1 B2 • • • Bi B(i+1) B(i+2) B(i+3) • • • Bn D1 D2 • • • Di D(i+1) D(i+2) D(i+3) • • • Dn
DATA (MSB) - DATA (LSB) PLUS OPTIONAL PARITY; D1 = DATA (MSB) Di = DATA (LSB) FOR NO PARITY Di = PARITY WHEN USED
STALE OVER FLOW
ALL REMAINING OVERHEAD Dn = (LSB)
DATA FIELD OVERHEAD FIELD
4.8.2.4 Stale Data Bit. A “stale data bit” flag shall be generated each time a new data value is inserted into the PCM stream. The flag shall be transmitted with the associated data. The flag bit shall be placed in the next less significant bit location following the LSB of the data. If new data is not ready for transmission by the time the PCM word must be sent again, either the old data or alternating one/zero fill shall be sent and the flag set. Stale data shall be indicated by a binary “one” (see Figure 4-6).
STALE BIT OVERFLOW BIT
0 0 FRESH DATA
0 1 DATA OVERFLOW 1 0 STALE DATA
1 1 USER DEFINED Figure 4-6 Overhead truth table.
4.8.2.5 Overflow Bit. An “overflow bit” flag shall be generated to indicate an abnormal condition in which data may be lost. The overflow bit shall be placed in the next less significant data bit location following the stale bit flag. An overflow bit at a binary “one” indicates that a data discontinuity exists between the current data word and the previous data word (see Figure 4-6 above).
4.8.2.6 Insertion Rate. The asynchronous word structure shall be inserted into the host PCM word at a rate to avoid data loss in the PCM stream.
****** END OF CHAPTER 4 ******
