VME slots

VMEbus Overview

VMEbus is a computer bus standard for rugged industrial computers and applications. Its form factors are based on Eurocard. VMEbus defines its own signaling system. It was first launched in 1981. Bus width of the original standard was 16-bits. Now the available standards are VME32 3Uand VME64 6U. VME32 3U means 32bit bus width in 3U eurocard size. VME64 6U means 64bit bus width in 6U eurocard size.

VMEbus Connectors

VMEbus architecture is based on separate address and data buses. In 32 / 32 bits implementation two different Eurocard connectors, P1 and P2, are used. The connectors allow full utilization of both the bus widths. P1 is based on three rows of 32 bits each. This implements 16 bits of data bus, 24 bits for address bus, and control signals. P2 occupies one row. This implements remaining 8 addresses and 16 data bits.

VME Arbitration Bus

Arbitration bus is a set of nine lines that controls the VME bus. One slot in Eurocard chassis is dedicated for arbitration. VME card contains arbiter module. When the card is inserted in the VME slot, card requests access to the VMEbus. This is done by lowering the request in line in the arbitration bus. When card frees the bus arbiter module checks other lines in the arbitration bus. If any line is low arbiter lowers the bus busy line and writes the card number on the bus grant out line.

VMEbus Write / Read Operations

For write operation arbiter bus contains two data strobe lines, one address strobe line, and one write line. When card writes data and address to the bus it pulls the address strobe, data strobes, write pins to low. Two data strobes lines indicate whether the data is 8 bit, 16 bit, 32 bit, or 64 bits. If data is transferred correctly, arbiter lowers data transfer-acknowledge line. In case of error bus error line gets lowered. The read operation is the same with read line low signal. This VMEbus signaling is asynchronous. VMEbus architecture also contains an interrupt bus. In case of an interrupt arbiter writes the interrupt level to the address bus. Moreover in VME all the transfers use DMA (direct memory access).

VXI bus

VXI is VME extension for instrumentation. Its features include additional time and trigger lines, mechanical standard protocol requirements, multi-chassis extension, message based communication, embedded and industrial application support. Chassis of VXI system contains 13 slots for the instruments to be attached. It also fulfills the power supply requirements for instruments. 6U VXI bus is commonly used. VXIpnp (VXI plug-n-play) products are also available in various hardware and software standards. The core market of VXI is Military and Avionics Automatic Test systems. Its value declined with the introduction of PXI (PCI extension for instrumentation).