The avionics systems on aircraft frequently contain general purpose computer components which perform certain processing functions, then relay this information to other systems. | DATA BUSSES INTRODUCTION The avionics systems on aircraft frequently contain general purpose computer components which perform certain processing functions then relay this information to other systems. Some common examples are the mission computers the radar processors RWRs and Jammers. Each system is frequently laid out as shown in Figure 1. The Input Output I O modules will vary in function but all serve the same purpose - to translate the electrical signals from one protocol to one of another in order to exchange information. I O modules are used similarly in general purpose computers MIL-STD-1773 GPIB HPIB HPIL etc. Figure 1. Avionics Block Diagram in laboratories to test equipment and or tie computers together via a local area network LAN to exchange information. Some of the methodologies include a star ring or bus type network see terminology at the end of this section . The high speed data busses on avionics computers do not operate as fast as the CPU clock speed but they are much faster than the interface busses they connect to. There are a number of interface busses which are widely used by aircraft avionics systems and test equipment. The most common include the RS-232 the RS-422 the RS-485 the IEEE-488 GP-IB HP-IB and the MIL-STD-1553A B. The MIL-STD-1773 bus is a fiber optic implementation of the 1553 bus and may be used in the future when technology requires it to reduce susceptibility to emissions or other reasons. A summary follows in Table 1 then a brief description of each follows immediately while a section covering each in more detail is provided later. Table 1. Summary of Bus Characteristics_See notes on next page Bus Max Length Max Number of Terminals 1 Type of Lines 2 Data Rate Rise Time 3 Data Format RS-232C 100 feet max 50 ft at 20k BPS 1 Serial 3-20 150 - 19 200 baud per sec 5- to 8- bit serial RS-422 km 4 10 5 Serial 3 see figure in RS-232 section Tb unspecified RS-485 unspecified 32 Serial 3 10 MHz Tb unspecified .
