This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
At least one device (SCSI card or any SCSI drive) must provide power for the SCSI terminators by feeding 5 volts onto the TERMPWR line on the SCSI bus. Usually, termination power is fed through a diode to prevent backfeeding from a higher voltage source in case some other device is also supplying termination power. A good implementation will have a fuse to protect against shorts and a capacitor to cope with a sudden rise in termination power drain. The Apple SCSI cards do not provide termination power (though some recent Apple Hi-speed SCSI cards were modified by Apple to provide termination power). The RamFAST SCSI card can supply termination power. If a drive can supply termination power, I recommend letting it do so. The TERMPWR line can, in some cases, represent a significant load on the +5V rail going to the Slots. Both of my Quantum drive mechanisms provide termination power to the SCSI bus, avoiding the need to supply it from anywhere else. ---------------------------- By: Rubywand On the RamFAST SCSI RevC card, DIP switch #1 is set to ON to supply termination power. On other RamFAST SCSI cards, a jumper is placed at JP1 to supply termination power. According to RamFAST documentation, it is okay to have the card set to supply termination power whether or not another device does with a few notable exceptions. If a connected hard disk (e.g. a Sider drive) has a sticker saying that the drive supplies termination power and that the interface must not, then the RamFAST must be set to not supply termination power. By: LJSilicon