The SCSI bus (small computer systems interface) is a high-speed bi-directional parallel bus used mainly for connecting ‘intelligent’ peripheral devices to a CPU. The bus is the de facto standard in the Apple computer environment, where it is used to connect devices like CD-ROM readers, scanners, and internal as well as hard disks. The SCSI bus has also found wide acceptance in the IBM PC world, where the main application is with large hard disks. SCSI hard disks feature a ‘software’ type identifier and require an appropriate hard disk controller and associated software.A SCSI bus network should have only one bus termination device. These devices are usually supplied with the PC or the SCSI peripheral, and consist of resistors only, connected between the TERMPWR (+5-V) line, the transmission (signal) lines, and ground
(SCSI drivers are open-collector types). The termination resistance ‘seen’ on every signal line is critical to ensure the highest possible data speed on the network. The circuit shown here is an active termination device, which eliminates the 220 Q and 330 Q resistors required for each signal line with a passive termination scheme, reducing significantly the continuous system power drain. When placed in series with 120 Q resistors, the L200 voltage regulator matches the imp,_. dance level of the transmission line, and eliminates reflections. Here. the L200 supplies a fixed voltage of about 2.75 V across Ri. The active termination device should replace the single. passive, termination device already fitted in the SCSI system. Construction is easy on the PCB shown. The voltage regulator, ICI, may be bolted directly to the PCB without a heat sink.
- Low-voltage differential (LVD) — A variation of HVD, but instead of the transceiver being built into the device, it is smaller and built into the SCSI adapter on each device so the voltage required for communication is also lower. Cheaper to implement compared to HVD but only has half the range at about 12 m.