Re: Hard Disk Woes

[dempson@actrix.gen.nz (David Empson)]

In article <Cuy3on.FCu@murdoch.acc.virginia.edu>,
Rolf Braun <rbraun@Hopper.ITC.Virginia.EDU> wrote:
> 
> The Apple II High-Speed SCSI card DOES provide ONE END of terminator
> power. You must still have a terminator plug at the other end of the chain
> in order to provide the OTHER end of termination power. My setup is:

No it doesn't.  Make sure you have your concepts sorted out.

"Termination" is a separate issue to "Termination Power".

"Termination" consists of a set of resistors in the device at each end
of the SCSI chain ("internal termination"), or attached after the
device ("external termination").

"Termination Power" means a device is actually providing a five volt
power source on the SCSI "TERMPWR" line, to power the termination
resistors at each end of the chain.  Termination power may be supplied
by any device on the SCSI bus - it must be supplied by at least one
device.  A diode is used in each device to prevent power feedback, so
nothing will be damaged if more than one device is able to supply
terminator power.  The one with the highest output voltage will "win".


The Apple SCSI card is terminated (i.e. it has termination resistors).
Originally, the card did NOT supply power for the termination.  Later
cards have a diode added so that the card will provide terminator
power, at least enough for its own terminators.  This can be retrofitted
to earlier HS SCSI cards.

The original Apple SCSI card doesn't have terminators, and doesn't
provide terminator power.

The RamFast has termination resistors.  It also has a switch which
selects whether or not the card should supply terminator power.


There is one issue which may be of concern: SCSI termination may draw
up to 480 mA from the five volt rail of the device supplying
terminator power.  If this is the computer, you may be overloading the
power supply, because the Apple II power supply is only rated to
supply 500 mA to ALL cards in the slots (i.e. total power drain from
all cards should not exceed 500 mA on the five volt rail).
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand