IEEE-488, VME and VXI - Computer Busses - page 336

First byte Second byte

DIO1-DIO8

NOT VALID VALID NOT VALID VALID DAV

NRFD

All devices ready
to receive data

NDAC

All devices have
accepted the
received data

Figure 22.3 GPIB handshaking

Interface management lines
The interface management lines are:

• ATN (attention) - this causes all the devices on the bus to interpret the data, either as a
controller command or as an address.
• IFC (interface clear) - bus reset.
• SRQ (service request) - used by a device to alert the controller that it requires to com-
municate.
• REN (remote enable) - enables devices to respond to remote program control.
• EOI (end or identify) - indicates the last transferred data byte of data.

22.3 VME bus

The VME computer bus is based on the IEEE-1014-1987 standard. Its main features are:

• A high-speed asynchronous data bus to transfer 8, 16 or 32 bits at a time.
• Four buses: data-transfer, arbitration, priority interrupt and utilities.
• Supports several bus controllers, such as the CPU, DMA, I/O controllers and any other
device that needs to control the bus. The arbitration bus avoids conflicts.
• Priority-interrupt buses where devices can request service from a VME interrupt handler
(similar to the service request [SRQ] line in IEEE-488).
• A utilities bus which provides power distribution, clocks, initialisation and failure detec-
tion.

The VXI (VME extension for instrumentation) is an extension to the VME bus, and is simi-
lar in its approach to the IEEE-488 bus. It is made up of several buses: VME bus, clock and
sync bus, the star bus, the trigger bus, the local bus, the analogue sum bus, the module
identification bus and the power distribution bus.