View Full Version : SCSI HD
Feb 21, 2005, 11:26 PM
I have a B&W G3, with a SCSI PCI card, and 2 SCSI HDs. I find SCSI drives pin placement a little more difficult then IDE drives. So here's the deal, I had one HD plugged in, and it worked great, I just added a second, and now it sees neither drive. Is there a reason for this? If the jumpers were wrong on one, would it make the other not work as well? What should the jumpers be set at to make this successful?
Feb 22, 2005, 12:29 AM
(reaching into the depths of antiquity)
SCSI needs 2 things: unique IDs, and termination.
A SCSI device can be any one of 7 address, ID#0 through ID#6 inclusive. IS#7 is reserved for the Mac motherboard. Each SCSI device on a given SCSI buss (or chain) must have a unique ID. The SCSI buss *usually* must be terminated at each end, and *usually* the SCSI controller on the motherboard is the terminator at that end. (Notice I am talking about SCSI Narrow (50 pin) here; the more exotic forms of SCSI have some differences in # of addresses and details of termination). By convention, the main internal hard drive is set to ID#0 and a CDROM is set to ID#3.
So, to add SCSI devices to a buss, first they must be set to unique IDs.
You usually have a set of three pairs of jumper pins for setting SCSI ID. Placing a jumper across a pair of pins selects that position
0 1 2
: : :
These correspond to binary values 000 through 111
All off = SCSI ID#0
postition 0 only = 1
position 1 only = 2 (binary 10)
position 1 and 0 = 3 (binary 11)
position 2 only = 4 (binary 100)
position 2 and 0 = 5 (binary 101)
postition 2 and 1 = 6 (binary 110)
position 2, 1 and 0 = 7 (binary 111) unused in Mac systems
SCSI devices may be added to the daisy chain in any position, the ID# is immaterial as long as you don't have 2 of the same ID on a single chain.
The last physical device on the chain or cable (regardless of its ID) must be terminated, which involves either adding a terminating resistor pack externally, or switching or jumpering a built in terminating resistor.
Hard drives often have a jumper labelled TE for termination. Jumpered is ON, no jumper is OFF.