13.2. Modular Instruments 727
to its flexibility and expandability, it has received its fair share of popularity in
industrial environments as well.
The original VME standard is now referred to as VME32 as it could handle up
to 32 bits of data. The new standard is called VME64 and has the capability of 64
bit data transfers. The salient features of VME64 are listed below.
64 bit data transfer and addressing modes.Two distinct data-address transfer modes: data transfer with the address and
address followed by the data.Multiplexed block transfer of data for faster delivery.40 bit addressing and 32 bit data transfer modes for P1-only systems.Jumper selection of address is not required any more.Another recent modification to the VME64 is the VME64x. This standard has
provided more flexibility and ease in operation with faster data transfer rates. Fol-
lowing are the distinguishing features of VME64x.
Addition of z and d rows to the P1 and P2 connectors bringing the total number
of pins to 160 per connector.User defined connections available on the rear side of the crate (backplane).Better grounding scheme.More DC voltage points for P1 and P2 connectors.A number of spare pins for future additions.Further modifications in these architectures have been done leading to faster data
transfer rates. For example, the so called2edgeVMEor 2eVME can essentially
double the peak block data transfer rate of standard VME64 and VME64x up to
160 MB/second. In the subsequent discussions we will be mainly concerned with
VME32 and VME64x.
C.1 VMELayout
VME was originally based on the Eurocard standard. A standard VME crate has
a maximum of 21 slots, out of which 20 can be used for inserting modules. The
first one is used by thecrate manager, just like thecrate controllerof the CAMAC
system. Typical VME crates are made with 20 slots so that they could fit into
standard racks. Crates with lesser number of slots are also available.
The height of a VME crate is measured in units ofUwith 1U =43. 60 mm.
Standard VME crate heights are 3U,6U,and9U. The standard modules orcards
are therefore available in these heights only. Similarly the two standard depths of
a VME crate are 160mmand 340mm. The thickness of a VME card should not
exceed 1.8mmsince then it might not slide in the rails.