Multidrop bus

A multidrop bus (MDB) is a computer bus in which all components are connected to the electrical circuit. A process of arbitration determines which device sends information at any point. The other devices listen for the data they are intended to receive.

Multidrop buses have the advantage of simplicity and extensibility. However, modern SDRAM chips exemplify the problem of electrical impedance discontinuity. Fully Buffered DIMM is an alternative approach to connecting multiple DRAM modules to a memory controller. Since 2000, multidrop standards such as PCI and Parallel ATA are increasingly being replaced by point-to-point systems such as PCI Express and SATA.

MDB in Vending Machines

The multidrop bus used by vending machine controllers to communicate with the vending machine's components, such as a currency detector, is also called MDB (for MultiDropBus). In use since the 1980s, it is now an open standard of the National Automatic Merchandising Association, or NAMA. The devices communicate in a single-master, multiple-slave configuration using the MDB protocol, which is based on a Motorola 9-bit UART 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. Though 9-bit compliant UARTs are not popular in PCs, they can be found in many microcontrollers.