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However, this distinction{{mdashb}}that power is provided by the bus{{mdashb}}is not the case in many [[avionics|avionic systems]], where data connections such as [[ARINC 429]], [[ARINC 629]], [[MIL-STD-1553B]] (STANAG 3838), and EFABus ([[STANAG 3910]]) are commonly referred to as “data buses” or, sometimes, "databuses". Such [[avionics#Aircraft networks|avionic data buses]] are usually characterized by having several equipments or [[Line-replaceable unit|Line Replaceable Items/Units]] (LRI/LRUs) connected to a common, shared [[Media (communication)|media]]. They may, as with ARINC 429, be [[Simplex communication|simplex]], i.e. have a single source LRI/LRU or, as with ARINC 629, MIL-STD-1553B, and STANAG 3910, be [[Duplex (telecommunications)|duplex]], allow all the connected LRI/LRUs to act, at different times ([[half duplex]]), as transmitters and receivers of data.<ref name = "ASSC 2003">Avionic Systems Standardisation Committee, ''Guide to Digital Interface Standards For Military Avionic Applications'', ASSC/110/6/2, Issue 2, September 2003</ref>
The frequency or the speed of a bus is measured in Hz such as MHz and determines how many clock cycles there are per second; there can be one or more data transfers per clock cycle. If there is a single transfer por clock cycle it is known as [[Single Data Rate]] (SDR), and if there are two transfers per clock cycle it is known as [[Double Data Rate]] (DDR). Within each data transfer there can be multiple bits of data. This is described as the width of a bus which is the number of bits the bus can transfer per clock cycle. <ref>{{cite book | url=https://books.google.com/books?id=j0wsBgAAQBAJ&dq=computer+bus+frequency&pg=PA39 | title=PC Systems, Installation and Maintenance | isbn=978-1-136-37442-5 | last1=Beales | first1=R. P. | date=11 August 2006 | publisher=Routledge }}</ref><ref>{{cite web | url=https://computer.howstuffworks.com/motherboard4.htm#:~:text=Bus%20speed%20usually%20refers%20to,dramatically%20affect%20a%20computer%27s%20performance | title=How Motherboards Work | date=20 July 2005 }}</ref> The data rate in bits per second can be obtained by multiplying the number of bits per clock cycle times the frequency times the number of transfers per clock cycle.<ref>{{cite book | url=https://books.google.com/books?id=6FnMBQAAQBAJ&q=Data+rate&pg=PA92 | title=Computer Busses | isbn=978-1-4200-4168-2 | last1=Buchanan | first1=Bill | date=25 April 2000 | publisher=CRC Press }}</ref><ref>{{cite book | url=https://books.google.com/books?id=vpnJDwAAQBAJ&q=Width | title=The Computer Engineering Handbook | isbn=978-1-4398-3316-2 | last1=Oklobdzija | first1=Vojin G. | date=5 July 2019 | publisher=CRC Press }}</ref> Alternatively a bus can use modulation such as PAM4 which groups 2 bits into symbols which are then transferred instead of the bits themselves, and allows for an increase in data transfer speed without increasing the frequency of the bus.<ref>{{cite web | url=https://www.xda-developers.com/pcie-6/ | title=PCIe 6.0: Everything you need to know about the upcoming standard | date=30 June 2024 }}</ref><ref>{{cite web | url=https://semiengineering.com/knowledge_centers/communications-io/off-chip-communications/pam-4-signaling/ | title=PAM-4 Signaling }}</ref><ref>{{cite book | url=https://
=== Bus multiplexing ===
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