Q: Kindly suggest me the online resources to read about MIL Std B protocol in detail. I am currently studying from MIL Std Designer’s guide by DDC. System Synchronization and ProtocolI Data Control I Subaddress Selection/Operation and. Data Storage. MIL-STD Tutorial (). Condor Engineering, Inc. Santa Barbara , CA MIL-STDB Defined. Chapter 3 Protocol. Word Types.

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MIL-STD is a military standard published by the United States Department of Defense that defines the mechanicalelectricaland functional characteristics of a serial data bus.

It was originally designed as an avionic data bus for use with military avionicsbut has also become commonly protoclo in spacecraft on-board data handling OBDH subsystems, both military and civil. Air Force standard inand first prootocol used on the F Falcon fighter aircraft. It is now widely used by all branches of the U.

The basic difference between the A and B revisions is that in the latter, the options are defined rather than being left for the user to define as required. It was found that when the standard did not define an item, there was no coordination in its use. Hardware and software had to be redesigned for each new application. The primary goal of the B was to provide flexibility without creating new designs for each new user. This was accomplished by specifying the electrical protocil explicitly so that electrical compatibility between designs by different manufacturers could be assured.


Six change notices to the standard have been published since Where a circular connector is used, its center pin is used for the high positive Manchester bi-phase signal.

Transmitters and receivers couple to the bus via isolation transformers, and stub connections branch off using a pair of isolation resistors and, optionally, a coupling transformer. This reduces the impact of a short circuit and ensures that the bus does not conduct current through the aircraft.

A Manchester code is used to present both clock and data on the same wire pair and to eliminate any DC component in the signal which protocoll pass the transformers. The bit rate is 1. The peak-to-peak output voltage of a transmitter is 18—27 V. The bus can be made dual or triply redundant by using several independent wire pairs, and then all devices are connected to all buses. There is provision to designate a new bus control computer in the event of a failure pdotocol the current master controller.

Usually, the auxiliary flight control computer s monitor the master computer and aircraft sensors via the orotocol data bus.

A different version of the bus uses optical fiberwhich weighs less and has better resistance to electromagnetic interference, including EMP. There may also be one or more Bus Monitors BM ; however, Bus Monitors are specifically not allowed to take part in data transfers, and are only used to capture or record data for analysis, etc. In redundant bus implementations, several data buses are used to provide more than one data path, i.

All transmissions onto the data 1553b are accessible to the BC and all connected RTs. Messages consist of one or more bit words command, data, or status. The 16 bits comprising each word are transmitted using Manchester codewhere each bit is transmitted as a 0.

Practically each word could be considered as a bit word: All communication on the bus is under the control of the Bus Controller using commands from the BC to the RTs to receive or transmit.

This means that during a transfer, all communication is started by the Bus Controller, and a terminal device cannot start a data transfer on its own. In the case of an RT to RT transfer the sequence is as follows: An application or function in the subsystem behind the RT interface e.

RT1 writes the data that is to be transmitted into a specific transmit sub-address data buffer. The time at which this data is written to the sub-address is not necessarily profocol to the time of the transaction, though the interfaces ensure that partially updated data is not transmitted.

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The Bus controller commands the RT that is the destination of the data e. RT2 to prptocol the data at a specified receive data sub-address and then commands RT1 to transmit from the transmit sub-address specified in the command. RT1 transmits a Status word, indicating its current status, and the data. The Bus Controller receives RT1’s status word, and sees that the transmit command has been received and actioned without a problem.

RT2 15533b the data on the shared data bus and writes it into the designated receive sub-address and transmits its Status word.

An application or function on the subsystem behind the receiving RT 15533b may then access the data. Again the timing of pritocol read is not necessarily linked to that of the transfer. The Bus Controller receives RT2’s status word proticol sees that the receive command and data have been received and actioned without a problem. If, however, either RT fails to send its status or the expected data or indicates a problem through the setting of error bits in the status word, the Bus Controller may retry the transmission.

Several options are available for 5153b retries including an immediate retry on the other data bus of a redundant pair of data buses and a retry later on the same bus in the sequence of transfers. The sequences ensure that the terminal is functioning and able to receive data. The status word at the end of a data transfer sequence ensures that the data has been received and pgotocol the result of the data transfer is acceptable.

However, the standard does not specify any particular timing for any particular transfer — that’s up to the system designers. Generally the way it is done on protoocol military aircraftthe Bus Controller has a schedule of transfers that covers the majority of transfers, often organized into a major frame or major cycle, which is often subdivided into minor cycles.

Similarly, there are four groups 3. Hence, where this scheduling structure is used, the prohocol are all at harmonically related frequencies, e. Whilst RTs cannot start a transfer directly on their own, the standard does include a method for when an RT needs to transmit data that is not automatically scheduled by the Bus Controller.

These transfers are often called acyclic transfers as they are outside the 1553b used by the cyclic executive.

MIL-STD Tutorial and Reference – Alta Data Technologies

In this sequence, an RT requests transmission through a bit in the status word, the Service Request bit. However, where an RT only has one possible acyclic transfer, the Bus Controller can skip this part.

The vector word is transmitted by the RT as a single bit data word. The format of this vector word is not defined in the standard, so the system designers must specify what values from what RTs mean what action the Bus Controller is to take.

This may be to schedule an acyclic transfer either immediately or at the end of the current minor cycle. This means that the Bus Controller has to poll all the Remote Terminals connected to the data bus, generally at least once in a major cycle. RTs with higher-priority functions for example, those operating the aircraft control surfaces are polled more frequently. Lower-priority functions are polled less frequently. MIL-STDB also introduced the concept of optional broadcast transfers, in which data is sent to all RTs that implement the option, but to which no RTs respond, as this would cause conflicts on the bus.

These can be used where the same data is sent to multiple RTs, to reduce the number of transactions and thus reduce the loading on the data bus. However, the lack of explicit responses by the RTs receiving these broadcasts means that these transfers cannot be automatically re-tried in the event of an error in the transaction. The Command Word is built as follows.


The first 5 bits are the Remote Terminal address 0— The sixth bit is 0 for Receive or 1 for Transmit. The next 5 bits indicate the location sub-address to hold or get data on the Terminal 1— Note that sub-addresses 0 and 31 are reserved for Mode Codes. The last 5 bits indicate the number of words to expect 1— All zero bits indicate 32 words. In the case of a Mode Code, these bits indicate the Mode Code number e. The Status Word decodes as follows.

The first 5 bits are the address of the Remote Terminal that is responding. The rest of the word is single bit condition codes. Some bits are reserved.

A ‘one’ state indicates condition is true; Message Error and Service Request are examples. More than one condition protoccol be true at the same time.

The image below exemplifies many of the protocol and physical layer concepts explained above. For example, the RT address contained in the Command Word has a value of 0x3 in range of 0 to The sixth bit is 1, indicating a Transmit from the RT. The sub-address is protovol The last 5 bits indicate the number of words to expect take a value of 1, which is matched by the single Data Word value 0x2 after the Status Word.

Protodol as explained above, devices have to start transmitting their response to a valid command within 4—12 microseconds. In the example, the Response Time is 8. The amplitude of the query is lower than the amplitude of the response because the signal is probed at a location closer to peotocol Remote Terminal.

In the Status Word, the first 5 bits are the address of the Remote Terminal that is responding, in this case 0x3.

It initiates all message communication over the bus. The B spec dictates that all devices in the system be connected to a redundant pair of buses to provide an alternate data path in prrotocol event of damage or failure of the primary bus. Bus messages only travel on one bus at a time, determined by the Bus Controller.

This may be used in ptotocol operation where handover occurs because of some specific function, e. Procedures for handover in fault and failure conditions generally involve discrete connections between the main and backup BCs, and the backup monitoring the actions of the main BC during operation.

For example, if there is a prolonged quiescence on the bus indicating that the active BC has failed, the next highest priority backup BC, indicated by the proticol connections, will take over and begin operating as the active BC.

MIL-STD – Wikipedia

A Bus Monitor BM cannot transmit messages over the data bus. Its primary role is to monitor and record bus transactions, without interfering with the operation of the Bus Controller or the RTs. These recorded bus transactions can then be stored, for later off-line analysis. Ideally, a BM captures and records all messages sent over the data bus. However recording all of the transactions on a busy data bus might be impractical, so a BM is often configured to record a subset of the transactions, based on some criteria provided by the application program.

This allows the Backup Bus Controller to “hit the ground running”, if it is called upon to become the active Bus Controller. For example, in a tracked vehicle, a Remote Terminal might acquire data from an inertial navigational subsystem, and send that data over a data bus to another Remote Terminal, for display on a crew instrument. Simpler examples of Remote Terminals might be interfaces that switch on the headlights, the landing lights, or the annunciators in an aircraft. The RT Production Test Plan is a simplified subset of the validation test plan and is intended for production testing of Remote Terminals.