10.6 Data Format Definition

10.6.4 MIL-STD-1553

10.6.4.1 MIL-STD-1553 Bus Data Packets, Format 0.

Reserved.

10.6.4.2 MIL-STD-1553 Bus Data Packets, Format 1.

MIL-STD-1553 BUS data is packetized in Message Mode, where each 1553 bus "transaction" is recorded as a "message." A four-item Intra-Packet Data Header is inserted prior to each transaction. A transaction is a BC-to-RT, RT-to-BC, or RT-to-RT word sequence, starting with the command word and including all data and status words that are part of the transaction, or a mode code word broadcast. Multiple messages may be encoded into the data portion of a single packet.

1. MIL-STD-1553 Packet Channel Specific Data. The Packet Body portion of each MIL-STD-1553 data packet begins with a Channel Specific Data word formatted as shown in Figure 10-17.

   msb 31 30	29 24	23lsb 0
   TTB	RESERVED	MSGCOUNT
   Message Count (MSGCOUNT). (Bits 23-0) indicate the binary value of the number of messages included in the packet. An integral number of complete messages will be in each packet. Reserved. (Bits 29-24) are reserved. Time Tag Bits (TTB). (Bits 31-30) indicate which bit of the MIL-STD-1553 message the Intra-Packet Header time tags. 00 = Last bit of the last word of the message 01 = First bit of the first word of the message 10 = Last bit of the first (command) word of the message 11 = Reserved

   Figure 10-17. MIL-STD-1553 Packet channel specific data format.
2. MIL-STD-1553 Packet Body. A packet with n MIL-STD-1553 messages has the basic structure shown in Figure 10-18. Note that the width of the structure is not related to any number of bits. This drawing is merely to represent relative placement of data in the packet.

   PACKET HEADER

   CHANNEL SPECIFIC DATA

   INTRA-PACKET TIME STAMP FOR MESSAGE 1

   INTRA-PACKET DATA HEADER FOR MESSAGE 1

   MESSAGE 1

   INTRA-PACKET TIME STAMP FOR MESSAGE 2

   INTRA-PACKET DATA HEADER FOR MESSAGE 2

   MESSAGE 2

   :

   INTRA-PACKET TIME STAMP FOR MESSAGE n

   INTRA-PACKET DATA HEADER FOR MESSAGE n

   MESSAGE n

   PACKET TRAILER

   Figure 10-18. MIL-STD-1553 Data packet, format 1.
3. MIL-STD-1553 Intra-Packet Header. After the Channel Specific Data, the MIL-STD-1553 data are inserted into the packet in messages. Each MIL-STD-1553 message is preceded by an Intra-Packet Header consisting of an Intra-Packet Time Stamp and an Intra-Packet Data Header.
   1. MIL-STD-1553 Intra-Packet Time Stamp. (8 Bytes) indicate the time tag of the MIL-STD-1553 message as follows.
      • The 48-bit Relative Time Counter that corresponds to the data bit indicated in the MIL-STD-1553 Channel Specific Data, Time Tag Bits (section 10.6.4.2a) with bits 31 to 16 in the second long word zero filled or;
      • The Absolute Time, if enabled by bit 6 in the Packet Flags (section 10.6.1.1g). Time format corresponds to the time format indicated by bits 2 and 3 in the Packet Flags (section 10.6.1.1g) and to the data bit indicated in the MIL-STD-1553 Channel Specific Data, Time Tag Bits (section 10.6.3.5.3).
   2. MIL-STD-1553 Intra-Packet Data Header. The length of the Intra-Packet Data Header is fixed at 6 bytes (48-bits) positioned contiguously, in the following sequence (Figure 10-19)

   msb 31lsb 0

   BLOCK STATUS WORD

   GAP TIMES WORD

   LENGTH WORD

   Figure 10-19. MIL-STD-1553 Intra-packet data header.
4. Block Status Word (BSW). (Bits 15-0) contain the Block Status Word for both the message type and whether any 1553 bus protocol errors occurred during the message transfer. The Block Status Word bit definitions are in Figure 10-20.

   15-14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
   R	BID	ME	RR	FE	TM	R	R	R	LE	SE	WE	R	R	R
   Reserved. (Bits 15-14) are reserved. Bus ID (BID). (Bit 13) indicates the bus ID for the message. 0 = Message was from Channel A 1 = Message was from Channel B Message Error (ME). (Bit 12) indicates a message error was encountered. 0 = No message error 1 = Message error RT to RT Transfer (RR). (Bit 11) indicates a RT to RT transfer; message begins with two command words. 0 = No RT to RT transfer 1 = RT to RT transfer Format Error (FE). (Bit 10) indicates a frame error 0 = No format error 1 = Format error Response Time Out. (Bit 9) indicates a response time out occurred. 0 = No response time out 1 = Response time out Reserved. (Bits 8-6) are reserved Word Count Error (LE). (Bit 5) indicates a word count error occurred. 0 = No word count error 1 = Word count error Sync Type Error (SE). (Bit 4) indicates an incorrect sync type occurred. 0 = No sync type error 1 = Sync type error Invalid Word Error (WE). (Bit 3) indicates an invalid word error occurred. 0 = No invalid word error 1 = Invalid word error Reserved. (Bits 2-0) are reserved.

   Figure 10-20. Block status word format

   Response Time Word (Gap). The Response Time Words indicate remote terminal response times as defined by MIL-STD 1553. The resolution of the response time shall be in tenths of microseconds. A maximum of two Response Time Words can exist. Remote Terminal to Remote Terminal type messages shall have two Response Time Words if both terminals respond, all other messages will have one Response Time Word, or none for broadcast type messages or messages with no Remote Terminal response.

   The Gap Times Word indicates the number of tenths of microseconds in length of the internal gaps within a single transaction. For most messages, only GAP1 is meaningful. It measures the time between the command or data word and the first (and only) status word in the message. For RT-to-RT messages, GAP2 measures the time between the last data word and the second status word. The Gap Times Word bit definitions are as shown in Figure 10-21.

   msblsb
   158	70
   GAP 2	GAP 1

   Figure 10-21. Response time word format.

   Gap measurements shall be made IAW MIL-STD-1553 response time measurements from the mid-bit zero crossing of the parity bit of the last word to the mid-zero crossing of the sync of the status word.

5. Length Word. The Length of the message is the total number of bytes in the message. A message consists of command words, data words, and status words.
6. Packet Format. Unless an error occurred, as indicated by one of the error flags in the Block Status Word, the first word following the Length should always be a command word. The resultant packets have the format shown in Figure 10-22.

   msb	lsb
   15	0
   PACKET HEADER
   CHANNEL SPECIFIC DATA (BITS 15-0)
   CHANNEL SPECIFIC DATA (BITS 31-16)
   INTRA-PACKET TIME STAMP FOR MSG 1 (BITS 15-0)
   INTRA-PACKET TIME STAMP FOR MSG 1 (BITS 31-16)
   INTRA-PACKET TIME STAMP FOR MSG 1 (BITS 47-32)
   INTRA-PACKET TIME STAMP FOR MSG 1 (BITS 63-48)
   INTRA-PACKET DATA HEADER FOR MSG 1 (BITS 15-0)
   INTRA-PACKET DATA HEADER FOR MSG 1 (BITS 31-16)
   INTRA-PACKET DATA HEADER FOR MSG 1 (BITS 47-32)
   COMMAND WORD
   COMMAND, STATUS, OR DATA WORD
   DATA OR STATUS WORD
   :
   DATA OR STATUS WORD
   INTRA-PACKET TIME STAMP FOR MSG 2 (BITS 15-0)
   INTRA-PACKET TIME STAMP FOR MSG 2 (BITS 31-16)
   INTRA-PACKET TIME STAMP FOR MSG 2 (BITS 47-32)
   INTRA-PACKET TIME STAMP FOR MSG 2 (BITS 63-48)
   INTRA-PACKET DATA HEADER FOR MSG 2 (BITS 15-0)
   INTRA-PACKET DATA HEADER FOR MSG 2 (BITS 31-16)
   INTRA-PACKET DATA HEADER FOR MSG 2 (BITS 47-32)
   COMMAND WORD
   COMMAND, STATUS, OR DATA WORD
   DATA OR STATUS WORD
   :
   DATA OR STATUS WORD
   :
   INTRA-PACKET TIME STAMP FOR MSG N (BITS 15-0)
   INTRA-PACKET TIME STAMP FOR MSG N (BITS 31-16)
   INTRA-PACKET TIME STAMP FOR MSG N (BITS 47-32)
   INTRA-PACKET TIME STAMP FOR MSG N (BITS 63-48)
   INTRA-PACKET DATA HEADER FOR MSG N (BITS 15-0)
   INTRA-PACKET DATA HEADER FOR MSG N (BITS 31-16)
   INTRA-PACKET DATA HEADER FOR MSG N (BITS 47-32)
   COMMAND WORD
   COMMAND OR DATA, WORD
   DATA OR STATUS WORD
   :
   DATA OR STATUS WORD
   PACKET TRAILER

   Figure 10-22. MIL-STD-1553 data packet, format 1.

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01/17/2007 23:34 - Bob (bob dot baggerman at gatech dot edu)

There is not enough info about the layout of command words, status words, and data. In a Ch 10 data file the order of command word(s), status word(s), and data words are the same as on the wire. In other words, the same order shown in the 1553 spec.

This really should be spelled out explicitly in this spec but isn't. This order, although reasonable I suppose, isn't universal. The UTMC 1553 parts, for example, put the command words, status words, and data words in fixed location offsets in memory. This is great because if I want to read them I don't have to calculate the offset into the data structure to get their location like with this Ch 10 spec. In other words, with the Ch 10 layout the location of the status word (and data in the case of RT to RT) needs to be calculated for every data packet. Annoying but not that big of a deal, .... if you know going in what the layout is.

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