Recently Ive been hearing about new high performance VME backplanes. How are they different from traditional VME backplanes and how will they enhance VME performance?
The VMEbus backplane provides the transport media for electrical signals between modules. As originally defined a VME backplane can contain up to 21 slots. Why was 21 slots selected? Because if you space connectors at 0.8 inch you can just get 21 slots into a chassis which will fit into a 19 inch rack. Backplanes can contain less than 21 slots and sizes such as 5, 7, 9, 12, and 15 slots are popular. For really small systems even 3 slot backplanes can be used. The VMEbus defines an asynchronous backplane protocol which uses handshaking signals rather than a clock to transfer data. Since the backplane is viewed as a transmission line, certain time delays are defined to assure that signals reach a known state before they are deemed valid. These time delays were determined by taking into account a fully load 21 slot backplane and characteristics of specific TTL logic available when the VMEbus was developed in the early 1980s. These delays have stood the test of time and have provided relia
