12.007 What is SCSI "termination power"?
Description
This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
12.007 What is SCSI "termination power"?
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
Continue to:
My Books
