markyb86,
The reason why there are terminators on SCSI is due to the fact that there is such a strong signal sent down the wire. When it reaches the other end, if nothing is there, it will bounce the signal back to the source (it echoes). It's almost like how echoes reflect off of the sides of mountains. Without a terminator, you will experience a variety of symptoms from "ghosting" (the icon for that SCSI drive appearing and disappearing repeatedly), "mirroring" (the icon shows up twice, one below the other) or no device at all. The terminator acts like a shock absorber for the entire chain.
One thing to remember about SCSI is that you can "daisy-chain" devices together:
Mac's SCSI port<===>SCSI device<===>SCSI device<===>SCSI device<===>etc.
This is why there are at least 2 SCSI ports on a SCSI device. SCSI, offers up to a handful of devices you can link together. But also in order for a successful set up, you must have assigned one SCSI ID number per device. Here's how it breaks down:
SCSI ID #0: Reserved for the Mac's first internal hard drive
SCSI ID #1:
SCSI ID #2:
SCSI ID #3: Generally reserved for internal and external CD drives
SCSI ID #4:
SCSI ID #5:
SCSI ID #6:
SCSI ID #7: Reserved internally for the Mac (not allowed to use)
Regarding SCSI, according to tech writers David Pogue and Joseph Schorr, there are "The 3 Rules of SCSI":
1.) Termination:
Typically, the Mac's internal hard drive will have its terminator enabled by default. Externally, regardless of what the SCSI device is, you must have a terminator installed on the last device on the chain. Now, some external SCSI devices may already have termination installed and (depending on the manufacturer) there may be a switch you can adjust to turn the terminator on or off. Also, some SCSI devices may have "active termination" built-in, where a circuit will turn on or off termination automatically as needed.
2.) SCSI addressing or ID number:
Each SCSI device must be assigned their own number and you can't share that ID number with another SCSI device. The physical order of devices can be independent of their ID numbers and the last device on the chain doesn't have to be the highest or lowest number assigned. However, Apple recommends you give higher numbers to the devices you use most frequently and lower numbers to devices used less frequently.
3.) Cabling:
Keep the cables as short as possible. You're allowed up to 18 feet (6 metres) total, but you must subtract from that figure the amount of cable inside an external device. Finally, the quality of the cables make a huge difference in the successful maintaining of SCSI. So if you can, buy cables from a name brand you can trust.
73s de Phreakout. :rambo: