CertiGuide to A+ (A+ 4 Real)  9  Chapter 10: The Small Computer System Interface, SCSI       9  Parallel SCSI Standards and Features

Tagged Command Queuing 1 Ultra160 SCSI

Ultra3 SCSI¹⁰³ is the next step in SCSI evolution. Ultra3 SCSI has a maximum transfer rate of 160 MB/sec using the same 40 MHz bus speed. The higher transfer rate is possible because the Ultra3 SCSI protocol is capable of trans-ferring data on both the rising and falling edges of the clock signals, for a maximum transfer rate of 160 MB/ sec with a wide 16-bit device.

Ultra3 SCSI includes all the features of Ultra2 SCSI, including multimode support. In addition, Ultra3 SCSI adds five features that differentiate it from Ultra2 SCSI:

• Double Transition Clocking: Data can be transferred twice per clock pulse on the asserting and negating edges of the clock pulse. This results in two bits of data being transmitted for each clock cycle. The maximum data transfer rate reaches 160 MB/sec without increasing the clock rate of the bus, which remains at the Ultra2 speed of 40 MHz.
  
  
• Cyclic Redundancy Check (CRC):¹⁰⁴ Previous SCSI implementations used par-ity checking to detect data errors. CRC can detect single-bit and double-bit errors, all odd number errors, and errors in burst transmissions up to 32 bits long. This is a huge improvement in error detection that helps maintain per-formance with marginal cabling or external device hardware and ensures data protection during hot plug operations.
  
  
• Domain Validation:¹⁰⁵ This feature checks the integrity of the entire SCSI bus, including cables, backplanes, and terminations, to ensure that data transfers can occur at the maximum possible rate. If a problem is encoun-tered, the data rate is reduced to a speed that the bus can reliably handle.
  
  
• Packetized SCSI:¹⁰⁶ Packetized SCSI improves data transfer rates in two ways: A packetized SCSI bus can send multiple commands simultaneously in packets. It also allows the ability to transfer commands at the maximum negotiated bus speed. This means that the SCSI commands can be sent at the 40 MHz bus speed. On a non-packetized SCSI bus, the SCSI commands are sent individually at 3 MHz.
  
  
• Quick Arbitration and Selection:¹⁰⁷ Quick arbitration reduces the bus idle time by allowing devices to arbitrate for the bus as required. With Quick Ar-bitration, the device informs the controller when it is ready to transfer data rather than waiting to be polled. Arbitration includes a fairness algorithm that prevents older devices from being excluded.

Tagged Command Queuing 1 Ultra160 SCSI