VME Rack or Chassis

- details of the basic VMEbus or VME rack or chassis with mechanical, physical and electrical details.

In many respects the VME chassis forms the basis for the whole VME system.

The VME rack or chassis is where the VME cards are contained. It also contains the backplane and all the elements such as power supplies, etc that enable the system to run.

As such the VME chassis is the central element for the whole system.Accordingly many manufacturers make VME chassis or VME racks as they are also sometimes called.

VME chassis basics

The VME chassis uses a standard Eurocard system as its basis.

The card cage contains 21 slots, the first of which is used for the VME rack manager.

There are two card sizes that are suitable:

  • 160 x 216 mm
  • 160 x 100 mm

Cards of both sizes may be mixed within the same VME rack.

The larger cards are acapable of performing 8, 16 or 32 bit transfers. They can also support a larger address range (4 Gbytes vs. 16 Mbytes). The smaller sized cards are able to support 8 or 16 bit transfers.

The two connectors that mate with the backplane are given the designations P1 and P2. All VME cards have the P1 connector while the larger cards may have the optional P2 connector.

In terms of the VME rack, some may support only the P1 connector whereas others support both. There is normally a cost saving for VME racks that support only the P1 connector.

VME rack elements

There are several elements that are contained within the VME rack of chassis:

  • VMEbus backplane:   The backplane consists of a printed circuit board on which the card connectors are located.

    Basic VME racks have a single connector and this runs on what is termed the J1 backplane. The connectors may be 96 or 160 pins.

    More comprehensive VME racks may have a second connector and connections for this are contained on a second backplane card on what is termed the J2 backplane.

    On some systems containing both the J1 and J2 planes, both will be contained on a single board as this saves costs in production.
  • VME cards:   These are the printed circuit boards that carry the required hardware to implement the functionality required on the system. They may be any one of the many forms of cards that have been developed for the VME system. These cards will have the P1 connector, and may also contain the P2 connector where additional I/O capability is needed.
  • Slots:   The slot is the term for the position where a VME card may be located within the backplane.
  • Subrack:   The VME sub-rack is the mechanical frame that supports the various backplanes and cards. It provides all the card guides, card locks, etc., to ensure that the cards and backplane mate satisfactorily. It also contains other functions such as cooling.

By Ian Poole

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