VME extension allows video transfers
With the advent of the PCI bus, CompactPCI, and PMC mezzanine boards, many systems integrators are taking a wait-and-see attitude on which bus structure to choose. For a while, it looked as if the venerable VME, even with its installed base and VME64X extensions, was going to play second fiddle to PCI. But at the Real-Time Computing Conference and Exhibition, held in Santa Clara, CA, in January, Bustronic (Fremont, CA) and Arizona Digital (Scottsdale, AZ) debuted a VME320 backplane capable of transferring data at a rate of 320 Mbyte/s between VME boards.
The two companies demonstrated real-time 1024 ¥ 768 video being transferred between prototype frame buffer and a display board under control of a 486-based CPU from Xycom (Saline, MI). Allowing synchronous data transmission, VME320 is more than five times faster than conventional VME64. "Unlike the `extended` VME64X, VME320 is compatible with existing VME boards and twice as fast," says Fred Hirsch, general manager of Bustronic.
Conventional VME does not use a synchronous protocol, but an interlocked one. Because of this, single data transfers require four times the propagation time of the bus, four times the logic delays, as well as added setup and hold times. VME64 doubles the standard VME peak data rate by using 32 address lines as additional data lines. VME64 hardware is compatible with the VME standard, except that master and slave boards must both support VME64 mode. To double data rates to 160 MHz, VME64X requires Enhanced Transceiver Logic (ETL) driver/receivers and five-row connectors and uses the positive-going edge of control signals.
According to Hirsch, "ETL transceivers and custom five-row, 160-pin connectors are expensive and perforate the backplane, resulting in substandard power transmission." But John Rynearson, technical director at VITA (Scottsdale, AZ), disagrees. "The 160-pin connector provides additional pins for new features and for user-defined I/O, besides being mandated for performance reasons.
Initial testing of VME320 boards by Bustronic confirmed that even removing all the cards from an existing VME system and transfer- ring them to a VME320 system resulted in no difference in the system`s operation. The VITA Standards Organization that is responsible for the VME64x draft standard is reviewing VME320 and developing a standard protocol to take advantage of the capabilities of the VME320 backplane. The group plans to have the initial draft of this protocol completed this month.
For more information contact Fred Hirsch at Bustronic, tel: (510) 490-7388, or Drew Berding at Arizona Digital, e-mail: 04152.23 07 @ compuserve .com, or John Rynearson at VITA, e-mail: TechDir@ vita.com.