||This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. (August 2013)|
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. Multidrop standards such as PCI are obsoleted by point-to-point systems, such as PCI Express.
MDB in Vending Machines
Multidrop buses are used by vending machine controllers to communicate with the vending machine's components, such as a currency detector (coin or note reader). These buses communicate with the MDB protocol, an 8-bit serial protocol with an additional mode bit. The mode bit differentiates between ADDRESS and DATA bytes.
MDB evolved as a standard protocol after 1995, allowing alternative payment systems (e.g. Smartcard based) 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 multiple vendors.
- IBM Journal of Research and Development
- MDB 3.0 (for vending machines) specification
- MDB 4.0 specification