A multidrop bus (MDB) is a computer bus in which all components are connected to the same set of electrical wires. A process of arbitration determines which device gets the right to be the sender of information at any point in time. The other devices must listen for the data that is intended to be received by them.
Multidrop buses have the advantage of simplicity and extensibility, but electronically are limited to around 200 - 400 MHz (because of reflections on the wire from the printed circuit board (PCB) onto the die) and 10 - 20 cm distance (SCSI-1 has 6 metres). Multidrop standards such as PCI are therefore being replaced by point-to-point systems such as PCI Express.
Multidrop buses are also used by vending machine controllers to communicate with the vending machine's components, such as a currency detector (coin or note reader). Not surprisingly, these MDB buses communicate with the MDB protocol, a 8-bit serial protocol with an additional mode bit. The mode bit differentiates between ADDRESS and DATA bytes.
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The MDB protocol is used in vending machines to interconnect different modules (such as bill acceptors, card readers and coin changers) with one bus. The physical connection is realized as an unmodulated serial bus with a fixed data rate of 9600 baud, and 9 bits in each data word transmitted.
MDB has evolved as a standard protocol after 1995, allowing alternative (e.g. Smartcard based) payment systems to be connected to existing vending machines.
Bus addressing is based on the device type, which allows for a very simple protocol stack as no initial enumeration needs to be performed, but has the disadvantage that only one device of each type can be attached.
The CCTalk multidrop bus protocol uses an 8 bit TTL-level asynchronous serial protocol, it uses address randomization to allow multiple similar devices on the bus (after randomisation the devices can be distinguished by their serial number). CCTalk was developed by CoinControls but is used by several other vendors too.
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