>>Apple's OEM CD drives are rather unique in that many of them use >selective
>>termination. With the term power jumped, the CD drive checks >the SCSI
>>chain and decides if it needs to have termination on or off. >It then
>>automatically sets itself. if the drive is at the end of the >chain, it
>>will terminate itself and if in the middle, turn termination >off.
>
>I would like to know where you heard this. I'm almost positive this is
>false. The TERM POWER jumper is to make a drive provide the termination
>power on the bus. At least one device on the bus must provide termination
>voltage.
>
>I continually say DO NOT trust a drive's onboard termination. You never know
>what the quality is. That is the reason the SCSI-3 spec dictates NO onboard
>termination at all.
>
>Internal devices in a Macintosh are tricky because the host adapter's
>termination is sometimes configured strangely. To be safe, get two discrete
>ACTIVE terminators and place them on the absolute ends of the bus (only if
>you have external devices attached, of course.) Make sure that at least one
>device provides term power.
>
>Again, narrow, slow SCSI is very forgiving. But as I heard on a list
>recently, "It's not that SCSI doesn't work when it should, but that it works
>when it shouldn't." But if you want to be certain, don't trust a drive's
>internal termination.
I would agree with most but not all of that - the 'term power'
option is not the ability to provide termination - this is provided by
the 'term enable' option pins IF the device has them. I have yet to
encounter an 80 pin drive with 'term enable' as these are usually
designed to fit arrays which already have a terminated chain.
Many 50 and 68 pin drives do have the 'term enable' option via a
jumper fitted to the drive and I see no reason to mistrust this option as
this is how all scsi macs and clones that I have used were terminated
when they left the factory. But I haven't experience of all macs...
Anybody have a scsi mac or clone with alternative standard
termination?
Pete
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