Multidrop bus
Encyclopedia
A multidrop bus 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 (because of reflections on the wire from the printed circuit board
(PCB) onto the die
) and distance (SCSI
-1 has ). 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.
s 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 , 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.
CCTalk was developed by CoinControls but is used by several other vendors too.
Computer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...
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 (because of reflections on the wire from the printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
(PCB) onto the die
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
) and distance (SCSI
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
-1 has ). Multidrop standards such as PCI
Peripheral Component Interconnect
Conventional PCI is a computer bus for attaching hardware devices in a computer...
are therefore being replaced by point-to-point systems such as PCI Express
PCI Express
PCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...
.
Multidrop buses are also used by vending machine
Vending machine
A vending machine is a machine which dispenses items such as snacks, beverages, alcohol, cigarettes, lottery tickets, consumer products and even gold and gems to customers automatically, after the customer inserts currency or credit into the machine....
controllers to communicate with the vending machine's components, such as a currency detector
Currency detector
A currency detector is a device that determines if a piece of currency is, or is not, counterfeit. These devices are used in vending machines that accept payment and dispense a product to a customer...
(coin or note reader). Not surprisingly, these MDB buses communicate with the MDB protocol
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
, a 8-bit serial protocol with an additional mode bit. The mode bit differentiates between ADDRESS and DATA bytes.
MDB in Vending Machines
The MDB protocol is used in vending machineVending machine
A vending machine is a machine which dispenses items such as snacks, beverages, alcohol, cigarettes, lottery tickets, consumer products and even gold and gems to customers automatically, after the customer inserts currency or credit into the machine....
s 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 , 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.
CCTalk
The CCTalk multidrop bus protocol uses an 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.
See also
- Bus network topology
- EIA-485
- Open collectorOpen collectorAn open collector is a common type of output found on many integrated circuits . Instead of outputting a signal of a specific voltage or current, the output signal is applied to the base of an internal NPN transistor whose collector is externalized on a pin of the IC. The emitter of the...