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It is highly recommended to know Why Data Buses are needed in the first place before reading about them.So why is it needed? a) It is needed to bring info from different sources to a central computer, a good example is our home computer where buses bring the data from the RAM, Hard Drive, Processor…….etc back and forth, in the same manner information from the Radar, Missile Warning Sytems, IRST…….etc is taken to the central computer for processing and the result is fed back to the Missiles, Micro wave link, radio…..etc and just like in the Home computer or in the Internet , faster buses in the jet means faster data transfer. b) The terms " Sensor Fusion, Data Fusion…..etc" (Though there is still a debate in circles on which is the apt term) is possible only by using the data bus. c) More still even the internal wirings of the equipments to the output central exchange is carried by the data bus, for example the Tarang RWR [ used in the MKI ] uses the MIL-STD-1553B bus and the ASR uses the H009 bus. d) In short " No DATA buses, No Exchange of data" What are the types of buses available? Though there are N number of buses available we shall stick with the most popular ones a) The MIL-STD-1553 A & B buses:This is the most popular bus used by the Third world countries fighter jets and even some of the USAF's jets uses these and is quite a decent bus, though the first world countries (leaving out Sweden) has moved on from the 1552. b) The MIL-STD-1773 : OFC version of 1553, no change in speed or the protocols, developed primarily to conform to Federal governments EM standards. c) The STANAG 3910: Faster bus supporting a low speed 1553 bus and a High speed Optical bus, with a structure similar to the ISDN (Note the term similar,They are not equal.) used in the EF2000 and the Rafale , though the un-fortunate stuff is that the speed has been reduced to permit the inter operatablity with the other NATO jets using the 1553. d) The H009 bus: Americans use N different buses [ ie one of the Raptor and JSF, One of the Comanche, One for the E, One for the C,one for the F-16……..etc] and many even now use the 1553 variants I have decided to club all the next generation American buses into the H009 family. Why such a number of buses? Requirements of different countries are different & hence the different types. MIL-STD-1553 Buses. MIL-STD stands for "Military Standard" and 1553 denotes the type of protocols and other structures associated with that standard. MIL-STD-1553 is the military standard for an aircraft's internal Time division command/Response multiplex buses, it was developed in the 1970's to over come the problems in integrating multiple avionics sub systems in military aircraft. The MIL-STD-1553 data bus is 1 Mbit/s serial data bus which provides integrated centralized system control and a standard interface for equipment/Sub system. With such data bus traffic on Data bus is accessible through a single connection for unto 32 users. A twisted shielded pairs of wires is used to interconnect these terminals. The first military aircraft using MIL-STD-1553 is the USAF F-16 and ARMY'S AH-64A in 1975. What are the Types in the 1553? The MIL-STD-1553 has two versions A and B, the major difference is options are predefined in B, where as in A it is upto the user to define it, generally all aircrafts uses the MIL-STD-1553B. Problems in the MIL-STD-1553: First the MIL is a serial, time division, multiplex command/response bus ie the information is transmitted in a single path with the allocation of resources to each users and all transactions can take place in response to a command from a central controller called as "Speak on when Spoken to system" and the system defines three systems which are a) Bus controller b) Remote terminal c) Bus monitor  The MIL-STD-1553 requires that signals be T'xd over the bus by using serial, digital, pulse code modulation with Manchester bi-phase level data code at a transmission bit rate of 1.0 megahz per second. Referring the above figure top wave form represents a 1 Mhz duty clock cycle while the middle waveform represents the data to be T'xd in NRZ format, lowest wave form represents the signal actually supplied to the bus a)firstly the waveform is self clocking b)waveform is designed to have a minimal DC component c) High noise immunity is achieved only be making the info contained in the signal transitions and levels relatively un-important This is the biggest drawback in the MIL bus since the bus is used to handle guiding a missile to its path by using its high speed micro wave links and suddenly a high priority message is reached and the priority is assigned to it , then the up-dation to the missile fails, perfectly justified if you are fighting in the 80's with the Jets having limited sensor capability and not having excellent flight characteristics coupled with the fact that the missiles like the Sparrow were not designed to get huge inputs it is quite okay, but what about today's missiles like the 120R or the Meteor or the soon to arrive ERAAM, they cannot even be guided if the 1553B or its successor the 1773 is used and is precisely the reason why the F-15 E's got the H009,the level of sensor fusion is actually de graded in the Grippen just due to the fact that the buses were not able to cope up with the message transfer taking place and required between the different systems and the main reason why AESA can never be used with anything near its full potential since it causes needless overloading of the communication channel and also the receiver. a) which does not permit the effective use of any modern AAM's b) With a speed of 730 kbps does not permit the transfer off video or any intensive data applications c) Utterly useless when an AESA antenna is used since all the info can never be fused d) Delay is huge in this bus. e) Due to command/Response protocol, MIL-STD-1553 buses have a deterministic nature of Data Transfer and permit prioritization of user's .Un fortunately the Communication efficiency decreases as message length and transmission speed increase in the command/Response data buses, which in turn causes communication latency. f) More over the use of a central computer is a weakness in such a protocol because failure of the Data bus can cause the collapse of data bus g) Further more Advanced Military Aircraft requires users to employ mission avionics multiplex , block transfer and vehicle management buses that require speeds of more than atleast 20 Mbbs and 730 kbbs. Thus is the bus still decent in the warfare of the 20th century, but SAAB is no better since they have developed a new controller [by Erricson and Texas], for the 1553 called ISBC to be used specifically for the Grippen and in the F-16 up-grade program What is the big deal of this new controller? a) A message is Transmitted only when it has been up-dated which is called as Transfer upon update detect (UPD&T) b) The message Transmission cycle is not constant . A new cycle of message transmissions is initiated as soon as the previous cycle is completed, thus there is no IDLE time which causes huge drop in performance in the MKI and in any jet which does not use our controller and the Transmission cycle is not synchronized to any common sub system. The ISBC is supported in the following manner: a) For messages of BC->RT the card ascertains if a message is up dated bit is a logical one and if so executes the instruction for data transfer. b) For messages of RT->BC or RT->RT type , every RT utilizes its vector word to indicate to the BC the update state of its message of its message and the BC , based on this information , performs message transmissions of the relevant messages only. Thus the improvement because of this new controller is a) Reduction of the average communication load by a factor of 3 to 4. b) Reduction of the average delay time by a factor of 3. c) Possibility of using the arrival of a specific message as an event , which signifies a specific action to execute. At present even the Grippen uses the previous thing, only the Israel F-16 uses this. The reason is that unlike previous controllers which are ASIC based this is a Pure DSP processor which must be fabricated according to requirements ie these specifically designed processors tailored to suite Individual type of jets with a clear understanding of all the different parts , this lack of Universal compatibility feature makes it quite a costly proposition but one which has to be undertook. Thus even in the 1553 which is useless in its current form has been "Sort Of" up-graded due to the tremendous base of Texas and erricsson but there is no need to even speak about buses since the Eagle uses the H009 bus and has no such problems in supporting and using all the weapons to its Fullest potential, end result Eagle wins before even the Weapon is fired. MIL-STD-1773: Compared with Co-Axial and Twisted pairs, Optical Fiber has the property of High BW, Low noise, Immunity to EMI, Light Weight and Small Size. In 1988 DOD issued a standard called as MIL-STD-1773 to promote OFC buses in their military equipments and all weapons developed after 1988 had to feature the 1773 (leaving united states) , Since 1773 was developed as a OFC variant of 1553 it has the same 1 Mbits/s serial Data bit rate.  |
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