STANAG 3910

History

The original STANAG 3910, i.e. the NATO standard, reached, at least, draft version 1.8, before work on it was abandoned in the early 1990s in favour of its publication through non-military standardization organizations: the foreword to Rev. 1.7 of the STANAG from March 1990 stated "The main body of this document is identical to the proposed Rev 1.7 of prEN 3910". Following this, several provisional, green-paper versions, prEN 3910 P1 & P2, were produced by working-group C2-GT9 of the Association Europeene des Constructeurs de Materiel Aerospatial (AECMA) (now ASD-STAN), before its development also ceased in 1996-7 (following the withdrawal of the French delegation, who held the chair of AECMA C2-GT9 at the time). As a result, the standard remains (as of Aug. 2013) in green paper form: the latest draft version is prEN3910-001 Issue P1, the front sheet of which states, 'This "Aerospace Series" Prestandard has been drawn up under the responsibility of AECMA (The European Association of Aerospace Industries). It is published on green paper for the needs of AECMA-Members.' However, despite this disclaimer, the document is offered for sale by ASD-STAN, currently (August 2013) at €382.64.

Utilisation

The incomplete nature of the standardization process (as of Aug. 2013) has not prevented at least two versions of STANAG 3910 being implemented: one for the Eurofighter Typhoon and one for the Dassault Rafale. The Eurofighter version, known as EFABus, is standardized by an internal Eurofighter document (SP-J-402-E-1039). The standardization documentation for the Dassault version is unknown.

The EFABus version of STANAG 3910 is known to use an electrical low speed (3838/1553B) control channel and a fibre optic HS channel. The version specified for the Dassault Rafale uses electrical media for both channels.

There are a number of manufacturers of avionic equipment that supply both flight and ground (e.g. test) equipment to this protocol standard.

Media

The (draft) standard contains annexes, known as slash-sheets, that specify a number of different media types for the high-speed and low-speed channels, implementations identifying a specific slash-sheet with the relevant specifications.

Optical

Versions of STANAG 3910 using optical media for the HS channel component require an additional passive component, in the form of an optical star coupler either reflective or transmissive, to interconnect the remote terminals. This limits the number of remote terminals that may be connected to the HS media, through the effect of the optical star on the optical power (determined by the number of "ways" of the star). Therefore, it may not be possible for all the (up to) 31 RTs (and 1 BC) that may be connected to the LS channel to have HS channel connections.

The optical media types include 200 and 100 μm diameter core (280, 240, or 140 μm cladding) Step-index profile (depressed cladding) optical fibre. These are much larger-core fibres than are commonly used in short-haul commercial applications, which are more normally 50/125 or 62.5/125 μm. This is, in part at least, to reduce the problems associated with contamination of the optical connectors – a given size of particle between the end faces of the fibre in a connector or misalignment of such a connector has significantly less effect on the larger fibre – which is seen as a significant issue in avionic applications, especially where contaminating environments, high vibration, and wide temperature ranges can apply.

The major difference between the transmissive and reflective star coupled fibre networks is that two fibres are needed with the transmissive star coupler to connect a line-replaceable item (LRI), but with the reflective star, and a "Y" coupler internal to the LRI, only a single fibre is required: a "Y" coupler is a three-port optical device that connects the simplex transmitter and simplex receiver to a single fibre that carries the optical signals transmitted and received by the LRI in opposite directions (half duplex). However, while the use of the reflective star reduces the cabling in the aircraft, and thus weight, the excess losses involved in the use of the "Y" couplers and reflective star coupler makes meeting the power budget requirements, given a transmitter power and receiver sensitivity, more difficult.

Whilst it is explicitly stated that the LS buses may be a fibre optic equivalent to STANAG 3838, e.g. MIL-STD-1773, there are no known implementations of this approach.

Electrical

Versions using an electrical HS channel require an additional active component, in the form of a "central repeater", with multi-tap collector and distributor lines (which use directional couplers to connect to the LRIs) and a buffer memory, to allow for small differences in data rates.

The standard and the electrical media slash sheet it contains specify a 100-ohm characteristic impedance cable for both collector and distributor lines. A maximum cable length is not given for either, and neither are limits on the numbers of directional couplers and thus RTs. However, the losses in the directional couplers, etc., especially for the RT furthest from the central repeater, and the limitations on dynamic range between the furthest (and most attenuated) and nearest (and least attenuated) RT, will limit the number of RTs operating to the standard that may be connected to the HS media.

System architectures

Since STANAG 3910 uses a 3838/1553B LS channel for control, the logical architectures that are supported are very similar to those described for 3838/1553B. Essentially, there is a bus controller (BC) and up to 31 individually addressed (0-30) remote terminals (RTs) connected to the bus. The BC then commands the RTs to receive or transmit the data, either as RT to RT, RT to BC, BC to RT, RT to RTs (broadcast), or BC to RTs (broadcast) transfers.

With electrical media HS buses, the physical architecture is like that with 3838/1553B, save that the central repeater has to be at one end of each of the collector and distributor lines: the RT's connections to these lines work preferentially in one physical direction along the bus - hence directional couplers.

The use of optical media for the HS buses, e.g. in EFABus, has a significant effect on the physical architectures: it is not practical to implement linier T coupled bus architectures, where the bus is run around the platform (e.g. the aircraft), and each line replaceable item (LRI) connects, though a stub, at the nearest convenient point in its path. Rather, each LRI has an optical physical media connection to a common star coupler, which passively connects it to all the other LRIs connected to the same star. In the case of a reflective star, the bus connection from the RT will be a single fibre cable, over which the RT both transmits and receives (half duplex). With a transmissive star, each RT is connected through two fibres, one for it to transmit and one for it to receive data over.

Transfer sequence

Transfers over the HS channel are initiated via the 3838/1553B LS channel, in an analogous way to the setup of 3838/1553B data transfers. 3838/1553B BC-RT transfers are sent to a specific subaddress of the receiving and transmitting RTs by the STANAG 3910 bus controller (BC). Despite this being a subaddress on the LS side of the RT, and thus exactly the same as any other 3838/1553B RT's subaddress, this subaddress is known as the "HS subaddress". The 3838/1553B BC-RT transfers each carry a single data word, known as an HS action word. Each HS action word identifies the HS message to be transmitted or received, analogous to the command words used to initiate 3838/1553B RT transfers. As with 3838/1553B transfers, there can be HS transfers from BC to RT, RT to BC, RT to RT, BC to RTs (broadcast) and RT to RTs (broadcast).

According to the standard, the HS actions words comprise the following:

As a 3838/1553B data word, the HS action word is preceded by the 3 bit-time data word sync field and followed by the single bit parity bit. As part of a 3838/1553B BC-RT transfer, it is preceded by a 3838/1553B command word, and should normally, i.e. if not broadcast, invalid, or illegal, elicit a 3838/1553B status word from the receiving RT.

In the case of an RT to RT HS transfer, the BC sends an HS action word to the receiving HS RT, instructing it to receive the HS message with a specified block count value at the specified subaddress. The receiving RT will then reply on the LS channel with an LS status word indicating it received the HS action word. The BC will then, after an intermessage gap on the LS channel, send another HS action word to the transmitting HS RT, instructing it to transmit the message, normally with the same block count value, and from one of its subaddresses. The transmitting RT will then reply on the LS channel with an LS status word indicating it received the HS action word and completing the HS control format. The HS RT transmitting an HS message will then begin its transmission within a maximum time measured from the parity (last) bit of the transmit HS action word. This initialization time is specified in the slash sheets, though all those in the current, draft standard are 24 to 32 μS. If the receiving HS RT does not receive the start of the HS message within a specified (in the slash sheet) time, which should be sufficient for the duration of the HS control format and the initialization time of the transmitter, it is required to timeout.

According to the standard, HS messages comprise the following: