VMEbus (VersaModular Eurocard bus) is a bus (computer data path) system, designed by Motorola, Signetics, Mostek, and Thompson CSF, that is used in industrial, commercial, and military applications worldwide. VMEbuses are used in traffic control systems, weapons control systems, telecommunication switching systems, data acquisition, video imaging, and robots. VMEbus systems withstand shock, vibration, and extended temperatures better than the buses used in desktop computers, making them ideal for harsh environments.
A VMEbus system is based on the VME standard. The VME standard defines the mechanical specifications such as board dimensions, connector specifications, and enclosure characteristics, as well as the electronic specifications for sub-bus structures, signal functions, timing, signal voltage levels, and master/slave configurations. The most recent VME standard is the VME64 standard. The VME64 standard specifies a 64-bit data path for 6U cards, a 32-bit data path for 3U cards, twice the bandwidth for data transmission, lower noise , and plug and play features. Since the VME64 standard, an extension called the VME64x was added that supports hot swap . VME64 cards can be used on older VME bus systems and older VMEbus cards can be used on VME64 systems.
In 1997, a modified VME bus architecture called the VME320 was released by Arizona Digital. This architecture is designed to increase data transfer to 320 Mbps and bandwidth to 500 Mbps. The backplane design is different from the original VMEbus backplane.
The VMEbus system uses Eurocards. A Eurocard is a European designed circuit board that uses a 96-pin plug instead of an edge connector making it more durable. There are three sizes: 3U which is 4 x 6 inches, 6U which is 6 x 12 inches, and 9U which is 14 x 18 inches. 3U cards support 8- and 16-bit data paths and 6U cards support 32-bit data paths. The VME standard does not support 9U cards. Each card is plugged into a backplane. A backplane can have up to 21 slots for cards. A VMEbus system is scalable and modular, which means a card can be added when needed without having to make any other changes to the system.
A real-time operating system ( RTOS ) is included when a VMEbus system is purchased. An RTOS is better for VMEbus systems because of their ability to do a task within a certain time limit. Non real-time operating systems can be used but are not recommended.
A VMEbus system uses a master/slave architecture. A master is a device that controls another device. For example, a computer sends data to a printer. The computer is the master, and the printer is the slave because the printer cannot control the computer. A VMEbus system may have several master devices, which is why it is called a multiprocessing bus.
A VMEbus system has four sub-buses: the arbitration bus, the data transfer bus, the priority interrupt bus, and the utility bus.
- The arbitration bus controls the requests from various devices using an arbiter module. It gives permission to each device to use the bus and notifies requesting devices when the bus is busy. Requests are based on priority. Requests that are the same in priority are daisy-chained. The arbiter module resides in slot 1 of the backplane.
- The data transfer bus is used for reading and writing operations between modules.
- The priority interrupt bus handles interrupt s and monitors the interrupt request lines, which range from interrupt request 1 to IRQ7. IRQ7 has the highest priority.
- The utility bus supports a system clock.