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Multi-Drop Bus / Internal Communication Protocol
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<h2 id="mechanism">Mechanism</h2> <p>The multidrop bus used by <a href="/facts/Vending_machine/3YNz3yKU">vending machine</a> controllers to communicate with the vending machine's components, such as a <a href="/facts/Currency_detector/v5u1yBtX">currency detector</a>, is also called MDB (for Multi-Drop Bus). In use since the 1980s, it is now an open standard of the <a href="/facts/National_Automatic_Merchandising_Association/PCU6faaf">National Automatic Merchandising Association</a>, or NAMA. The devices communicate in a single-master, multiple-slave configuration using the MDB <a href="/facts/Communications_protocol/5uady7cB">protocol</a>, which is based on a <a href="/facts/Motorola/ztqczx68">Motorola</a>[<i>specify</i>] 9-bit <a href="/facts/UART/xMntJcsI">UART</a> implemented as an 8-bit data value with an additional mode bit. The mode bit differentiates between ADDRESS and DATA bytes. The master sends messages containing one address byte and a variable number of data bytes. The bus "slave devices" listen for an address, and if it matches their address that slave device will process the message and respond to the master.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> Though 9-bit compliant <a href="/facts/UART/xMntJcsI">UARTs</a> are not popular in PCs, they can be found in many <a href="/facts/Microcontrollers/MKrnAhnm">microcontrollers</a>. </p><p>The physical connection is realized as a serial <a href="/facts/Network_topology/rBYE0kke">bus</a> with a fixed data rate of 9600 <a href="/facts/Baud/uCsmXrdT">baud</a>. There are just 2 communications signals plus the essential common-ground reference signal. The TX signal goes from the MASTER to every SLAVE device. The RX signal goes from every SLAVE device to the MASTER device. Both signals have <a href="/facts/Pull-up_resistor/zeEjjrqk">pull-ups</a>. The bus is driven at every transmitter by an <a href="/facts/Open_collector/E12DL8fL">open collector</a> transistor driver, and isolated at each receiver with an <a href="/facts/Opto-isolator/cTVnQkhN">opto-isolator</a> - though cable harnesses carrying the communication signals may also carry 24-volt power and ground signals to devices, meaning the devices may not be isolated from each other as they share the same power bus. Some devices, however, may have alternate power supplies, especially devices with motors and high current needs such as vintage bill acceptors or <a href="/facts/Currency_detector/v5u1yBtX">currency detector</a> devices. </p> <h2 id="history">History</h2> <p>MDB originated as a proprietary bus used by CoinCo for their coin-acceptors in the late 1980s and was deployed in high volume in vending machines for <a href="/facts/Coca-Cola/wbnCJYb6">Coca-Cola</a>. Coke forced CoinCo to <a href="/facts/Open-source_software/HzTaB8E9">open-source</a> it in 1992 to increase competition, and NAMA released the first version of the standard in 1995, allowing other vendors to compete for the coin-acceptor portion of the vending machines (CoinCo and Mars were the 2 major suppliers in North America at the time) and also enabled alternative payment schemes (e.g. <a href="/facts/Smartcard/K7b9lowS">Smartcard</a> based) to be connected to existing vending machines. </p><p>Bus addressing is based on the device type only, which allows for a very simple protocol stack, as no initial enumeration needs to be performed. </p> <h3>Timeline</h3> <ul><li>August, 2019: Version 4.3 released (seventh MDB release)<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a></li> <li>February, 2011: Version 4.2 released (sixth MDB/ICP release)<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a></li> <li>July, 2010: Version 4.1 (fifth MDB/ICP release)</li> <li>April, 2009: Version 4.0 (fourth MDB/ICP release)</li> <li>March, 2003: Version 3.0 (third MDB/ICP release)<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a></li> <li>October, 2002: Version 2.0 (second MDB/ICP release)</li> <li>October, 1998: Version 1.0 (first MDB/ICP release)</li> <li>1994: Revised EVMMA document.</li> <li>August, 1994: Revised NAMA document.</li> <li>1994: Original EVMMA document.</li> <li>October, 1993: Original NAMA document.</li> <li>Earlier US versions were known as MDB rather than MDB/ICP. Prior to this, it was allegedly a proprietary protocol developed by CoinCo.</li></ul> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/National_Automatic_Merchandising_Association/PCU6faaf">National Automatic Merchandising Association</a></li> <li>European Vending Association</li> <li>European Vending Machine Manufacturers Association</li> <li><a href="/facts/Multidrop_bus/2SMRsccq">Multidrop bus</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="https://github.com/MarginallyClever/MDB-Sniffer">MDB-Sniffer</a></li> <li><a href="https://github.com/perdidor/Arduino-MDB-UART">MDB-RS232 adapter</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"Multi-Drop Bus / Internal Communication Protocol - MDB / ICP - Version 4.2" (PDF). <a href="https://www.ccv.eu/wp-content/uploads/2018/05/mdb_interface_specification.pdf" target="_blank">https://www.ccv.eu/wp-content/uploads/2018/05/mdb_interface_specification.pdf</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"Overview of Vending Machine Interfaces" (PDF). European Vending Association. 2012. <a href="http://www.peraltis.lt/uploads/images/overview-of-vending-machine-interface_2012.pdf" target="_blank">http://www.peraltis.lt/uploads/images/overview-of-vending-machine-interface_2012.pdf</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"MDB Specification Version 4.2" (PDF). NAMA. Archived from the original (PDF) on 2015-06-27. Retrieved 2018-10-11. <a href="https://web.archive.org/web/20150627103226/http://vending.org/images/pdfs/technology/mdb_version_4-2.pdf" target="_blank">https://web.archive.org/web/20150627103226/http://vending.org/images/pdfs/technology/mdb_version_4-2.pdf</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"MDB v4.3" (PDF). <a href="https://www.namanow.org/wp-content/uploads/Multi-Drop-Bus-and-Internal-Communication-Protocol.pdf" target="_blank">https://www.namanow.org/wp-content/uploads/Multi-Drop-Bus-and-Internal-Communication-Protocol.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"Multi-Drop Bus / Internal Communication Protocol - MDB / ICP - Version 4.2" (PDF). <a href="https://www.ccv.eu/wp-content/uploads/2018/05/mdb_interface_specification.pdf" target="_blank">https://www.ccv.eu/wp-content/uploads/2018/05/mdb_interface_specification.pdf</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"Multi-Drop Bus / Internal Communication Protocol - MDB / ICP - Version 3.0" (PDF). Archived from the original (PDF) on 2018-05-17. <a href="https://web.archive.org/web/20180517054259/http://www.technical.vending.org/MDB_3.0.pdf" target="_blank">https://web.archive.org/web/20180517054259/http://www.technical.vending.org/MDB_3.0.pdf</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> </ol>