728 Chapter 13. Data Acquisition Systems
C.2 VMEBackplane
The backplane of a VME crate is generally referred to aschassis. The chassis
contains connectors for the VME cards and has different buses for data, addresses,
signals, and power. Following are the buses found in standard VME backplanes.
Utility Bus:VME32 has +5V and± 12 Vsupplies. VME64 has in addition
a3.3V supply as well. These voltages are supplied to the modules through
the utility bus. Other lines present on this bus are system clock, system reset,
serial data, system failure, and AC failure.Data Bus:All the data transfer between modules takes place through this bus.
The width of the bus depends on whether the system is VME32 or VME64.
Data bus also contains data strobe lines. These lines contain the information
about the availability and size of the data. The read and write operations are
distinguished by the state of a dedicated WRITE line of the data bus. The
data transfer error signal is carried by a bus error line and the data transfer
completion is signaled by a transfer acknowledge line.Address Bus:Each module in a VME crate is identified by a unique address
of its register. A module can therefore be accessed by invoking its register
through the address bus line. The length of the address and the type of data
cycle are carried by the address modifier lines.Interrupt Bus: The interrupt request lines allow initiation of a new data
cycle.C.3 VMEModules
A large number of standard VME modules are readily available in the market. With
these off-the-shelf units one can essentially develop a whole data acquisition system.
However many researchers prefer to develop customized modules according to their
own requirements. This has also led to the development of non-standard VME
backplanes. These backplanes are based on the basic VME principles but differ in
bus topology and size.
The standard VME backplanes have specific connectors to which the modules
must conform. The two most commonly used connector types are referred to as
P1 and P2. The reader should however note that the pin assignments of these
connectors for VME32 are different from those of VME64x. The VME32 P1 and
P2 connectors have 3 rows of 32 pins each with a total of 96 pins. On the other
hand the VME64x connectors have 5 rows of 32 pins each with a total of 160 pins.
The pin assignments of these connectors can be found in Appendix C. Note that P1
connector is mandatory irrespective of the module size. Other connector types are
also available, such as 95 pin P0 connector, which has 5 rows of 19 pins each.
C.4 VMELogic
VME is a TTL based system. That is, its internal functioning follows TTL logic.
However modules accepting and producing other logic signals can be integrated into
the system.