Out in Front: September 2, 1996
A new SCSI I/O processor promises peak data-transfer rates of 80 Mbytes/sec yet retains compatibility with slower "legacy" SCSI modes. Symbios Logic's SYM-53C895 PCI to Wide Ultra2 controller achieves the higher rates by incorporating low-voltage-differential (LVD) transceivers in the controller itself, rather than requiring external transceivers. Optionally, however, the chip can use external transceivers for slower SCSI modes, such as high-voltage-differential operation, thus providing backward compatibility with existing Fast SCSI and Ultra SCSI controllers.
The new chip's LVD transceivers are key to the high data rates. With high-voltage-differential signals, transceivers must be external to a SCSI controller to avoid problems of high on-chip power consumption. And, because the transceivers are external, they introduce skew problems. A "pessimistic" skew, obtained by subtracting the minimum from the maximum transceiver delay over temperature, VCC, and process variations of the multiple ICs, is typically 6 nsec, which cuts into transfer time and thus reduces data rates. When all functions are on the same die, however, which LVD technology and its reduced power consumption make possible, a typical skew time is less than 1 nsec.
The LVD technology used in Symbios' new SCSI controller is based on the TIA/EIA-644 standard. Disk-drive manufacturers, system integrators, and suppliers of SCSI termination devices are currently testing the technology. The testing goal is validation of reliable 80-Mbyte/sec data transfers over 12m cables to 16 SCSI devices. The SYM53C895 controller costs $47 (5000).
—by Stephen Kempainen
Symbios Logic, Fort Collins, CO. (800) 856-3093, http://www. symbios.com.