GB/T 43669-2024Space data and information transfer systems - 1553B data bus communication protocol on spacecraft (English PDF)

1 Scope

The document describes the bus topology, the usage conventions, the bus communication mechanisms, the bus communication processes, the reliability design and the management information base of 1553B bus communication as applied on spacecraft.

It applies to the development of the equipment and systems that use the 1553B bus on spacecraft and of the matching ground test equipment.

2 Normative references

One normative reference is listed: GB/T 42041, Astronautics terminology - Space data and information transfer. For dated references only the edition corresponding to that date applies; for undated references the latest edition, including all amendments, applies.

3 Terms and definitions

The terms and definitions given in GB/T 42041 and the following ones apply to the document.

3.1 1553B data bus: a digital, time-division command/response type multiplex data bus.

3.2 terminal: the electronic assembly that connects the data bus to an application device. Note 1: on the 1553B bus there are three kinds - bus controller, bus monitor and remote terminal. Note 2: a terminal may exist as a separate replaceable assembly or may be contained inside the application device.

3.3 bus controller (BC): the terminal that organizes information transfer on the 1553B bus.

3.4 bus monitor (BM): the terminal that receives and records the messages transmitted on the 1553B bus and is able to extract information selectively. Note: if it is assigned a terminal address and takes part in communication, it is called a monitor terminal (MT).

3.5 remote terminal (RT): every terminal on the 1553B bus that takes part in communication without acting as bus controller or bus monitor.

3.6 word: the basic data unit of 1553B bus communication. Note 1: a word is made up of 20 bit, comprising a 3 bit synchronization header, 16 bit of data and 1 bit of odd/even parity; the 3 bit synchronization and the 1 bit check serve the physical layer, and the word that carries the information is the 16 bit of data. Note 2: there are three types of word - command word, status word and data word.

3.7 message: the basic unit of the data transferred on the bus. Note: it includes the command word, the status word, from 0 to 32 data words and the status response intervals, within one transmission sequence.

3.8 message frame: a message sequence made up of several messages. Note: the bus controller arranges the order of the bus communication messages by organizing message frames.

3.9 mode code: the class of message by which the bus controller manages the information flow of the bus system and the associated hardware, and which is not used for data transfer.

3.10 broadcast: a manner of bus communication in which a message sent by one terminal can be received by several other terminals or by all terminals.

3.11 redundant bus: a data transfer path that uses more than one bus between terminals.

3.12 distribution: the process by which the bus controller sends out a message.

3.13 acquisition: the process in which the bus controller organizes a remote terminal to send out a message and other remote terminals or the bus controller act as the receiving side of that message.

3.14 set data: the operation by which an application taking part in bus communication hands the data to be sent over to the terminal for transmission.

3.15 get data: the operation by which an application taking part in bus communication extracts the received data from the terminal.

4 Abbreviated terms

The abbreviations used in the document are BC for bus controller, BCA for BC application, BM for bus monitor, OSI for open system interconnect, RT for remote terminal and RTA for RT application.

5 General principles

5.1 Protocol layers and content. Figure 1 shows how the 1553B bus communication protocol specified in the document corresponds to the OSI layered protocol. The 1553B bus communication layers on a spacecraft cover the physical layer, the data link layer and the protocol of the interface towards the application layer. From the application point of view the document mainly specifies how the 1553B bus link layer protocol is used; it corresponds to part of the content of the link protocol sublayer and to part of the interface content at the lower end of the application layer, chiefly the bus topology, the subaddress conventions, the selection of mode code messages, the bus communication mechanisms and the bus communication processes. These serve the arrangement of messages and the exchange of data during 1553B bus communication, keep bus communication stable and reliable, and support the drawing up of specific mission protocols at the upper layer.

5.2 Bus topology. A 1553B bus system consists of one BC and of 1 to 31 RT (addresses 0 to 30); a BM may be configured as required. The topology is shown in Figure 2. The BC and the other terminals form a master-slave communication relationship and are connected by bus cables. Dual redundant bus cabling is normally configured, and multiple redundant cabling may be configured according to need and importance; the cables back each other up and a bus communication message may be transmitted over any one of them. Every terminal has a corresponding service application object, which may belong to the same device as the terminal or may be a replaceable assembly outside that device. The BM mainly monitors the state of bus communication and hands the monitored data over to the BM application for processing; it can also take part in normal communication by being assigned a terminal address, in which case it is called a monitor terminal (MT). The BM can act as a backup of the BC: when it monitors abnormal BC operation it can switch to BC mode and work in place of the BC. The switchover can be carried out automatically by some mechanism or through an external command; when switching over, the BC working mode of the BC is normally stopped first and the BM is then switched to BC working mode, so that two BC do not work on the bus at the same time. There are 32 RT addresses, of which 0 to 30 (binary 0 to 11110) are the exclusive addresses of the RT and 31 (binary 11111) is used for broadcast; in practice 0 should as far as possible not be used as the exclusive address of an RT.

5.3 Subaddress conventions. Each RT has 32 transmit subaddresses and 32 receive subaddresses, used to distinguish the sending and receiving of different kinds of data message. The use of the RT subaddresses is fixed in Table 1, and the explanation of the definitions together with the recommendations for use are given in A.3. Table 1 has four columns - subaddress, RT transmit, RT receive and remarks - and reads as follows. Subaddress 0 is not used. Subaddresses 1 to 26 carry acquire data on transmit and distribute data on receive; the kinds of message may be defined according to the needs of the mission, and they serve the send and receive processes (7.4, 7.5) as well as data block transfer (A.5). Subaddress 27 carries the distribution transfer acknowledgement on transmit and the distribution transfer description on receive, for the handshake send process between the sending and the receiving side (7.4). Subaddress 28 carries the acquisition transfer request on transmit and the acquisition transfer acknowledgement on receive, for the handshake receive process between the sending and the receiving side (7.5). Subaddress 29 carries the time code on transmit and the time code on receive, where the receive direction may also be broadcast; it serves the time distribution process (7.8). Subaddress 30 carries the long loop-back test in both directions, for the long loop-back test process (6.5.1, 7.9.1). Subaddress 31 carries the mode code in both directions, whose use is agreed in 5.4.

5.4 Conventions for the use of mode code messages. The 1553B bus defines 15 kinds of mode code message; the document fixes the way the following five commonly used mode code messages are used and lays down no usage convention for the remaining ten. a) Synchronize (binary code 00001): without data word, broadcast or non-broadcast; used to notify the relevant RT of a predetermined event so that the RT synchronizes; the synchronization process is given in 6.3 and 7.7 and the time distribution process in 6.4 and 7.8. b) Initiate self-test (binary code 00011): without data word, non-broadcast; used to start the test circuit inside the RT; the test process is given in 6.5.2 and 7.9. c) Transmit vector word (binary code 10000): with one data word, called the vector word, non-broadcast; used to require the RT to send the vector word so that the BC can determine the particular message transfer request of that RT; the service request mechanism is given in 6.2 and the definition of the vector word in A.2. d) Synchronize with data word (binary code 10001): with one data word whose coding can define different predetermined events, broadcast or non-broadcast; used to notify the relevant RT of a predetermined event so that the RT synchronizes on that event; the synchronization process is given in 6.3 and 7.7 and the time distribution process in 6.4 and 7.8. e) Transmit self-test word (binary code 10011): with one data word, called the self-test word, non-broadcast; used by the RT to report the result of the self-test to the BC; the test process is given in 6.5.2 and 7.9.

6 Bus communication mechanisms

6.1 Message transmission mechanism. The BC arranges and organizes message frames in advance according to the message transfer needs of the BCA and of the RT, and controls the transfer of the message frames through start and stop operations. There are two message transmission mechanisms, cyclic transmission and inserted transmission. Bus communication normally uses the cyclic transmission mechanism, sending messages continuously in cycles with the message frame as the unit. When a burst message has to be sent, the transfer of the cyclic message frame is stopped, the burst message is sent, and the cyclic message frame is then resumed at the point where it stopped; this changeover process is called the inserted transmission mechanism. The cyclic transmission mechanism is the advance allocation of bus communication bandwidth to the messages that have a cyclic transfer need, so that every terminal has the chance to take part in communication and the differing transfer delay requirements of the various messages are met; it gives bus communication its cyclic character and its timing stability. The inserted transmission mechanism serves the messages that have a non-cyclic transfer need and secures the timeliness of their transfer; the BC arranges and organizes the transfer according to the agreed priority relations and timeliness requirements of the burst message transfer requests of the BCA or of an RT. The message arrangement for the cyclic transmission mechanism and for the inserted transmission mechanism is given in A.1.

6.2 Service request mechanism. The service request mechanism allows bus communication to transfer messages on demand, so as to save transfer bandwidth and raise transfer efficiency. Data transfers in which an RT is the message source, or in which an RT requests to acquire a class of message, may use the service request mechanism. The RT marks the message transfer request by means of the service request bit in the status word of the transmit vector word mode code message (10000), and marks the kind of message whose transfer is requested by means of the vector word it carries. The BC cyclically issues the transmit vector word mode code message (10000) to each RT in turn; if the service request bit in the status word returned by an RT is 1, that RT has a message transfer service request, and the BC then analyses further the vector word returned by that RT, determines the kind of message whose transfer the RT requests, and organizes the transfer. The 16 bit vector word may define the kind of message bit by bit or by coding, as given in A.2. Where the vector word defines the kind of message bit by bit, the RT sets the corresponding bit of the vector word to 1 and the service request bit in the status word to 1 once the data are ready in the send buffer or when it requests to acquire a kind of message; when the transfer of the requested message is finished it sets the corresponding bit back to 0, and if all the bits of the vector word are 0 there is no further message transfer service request and the service request bit is set to 0. Where the vector word defines the kind of message by coding, the RT sets the vector word to the required code and sets the service request bit in the status word to 1; when the transfer of the requested message is finished it sets the vector word to the code of the next requested kind of message, and if there is no further message transfer request it sets the vector word to all zeros and the service request bit to 0.

6.3 Synchronization mechanism. There are three ways of synchronizing: the synchronize mode code message (00001), the synchronize with data word mode code message (10001), and non-mode-code synchronization messages. The synchronize mode code message (00001) and the synchronize with data word mode code message (10001) act on the synchronization of a given bus communication state, and the data word carried by the synchronize with data word mode code message (10001) can be coded so as to indicate different synchronization events. Both mode code messages can work together with the time code message, so as to transmit time on the bus and let the receiving side correct itself, as given in 6.4. A non-mode-code synchronization message is an ordinary message agreed between the parties taking part in communication; it can be used for the synchronization of data updating when an RT sends and receives data, and can also act as the mark separating the current bus cycle from the next one, as given in A.1. Note: the synchronization method actually used and its agreed meaning are determined by the mission protocol and are not specified in the document.