Is the zip termination active or passive? Don't know, but it is probably active. The specsheet on the IOmega website does not specify which it is.
Would you need to terminate the zip on the end of the chain? Only if it is the last device. Whatever the last device is, this is what you need to terminate.
Is the scanner terminator add-on active or passive? Who knows - post some specs or a part number and someone will try to find out. Or just google it.
Is the length of scsi external units too long? I don't know, but in my experience there aren't any hard and fast rules, just general guidelines. If it works, great. If not, reconfigure and try again. :-)
Regarding your current issues I can see a couple of possibilities:
1) Perhaps the termination power is insufficient with so many devices attached. You could activate the TERMPWR jumper on your harddrive or you zip disk (probably a good idea anyway with so many devices attached.
2) Re-reading your original post, it sounds as if you haven't tried with
just the scanner attached. Perhaps this will shed some light on the weak link in your scsi chain.
3) In post #3 you state that the scanner
uses a passthrough centronics type terminator
It sounds as if this is not a terminator - there is no reason for a terminator to have a pass-through, as by definition the terminator marks the end of the chain. Perhaps it is just a centronics <> d-sub adapter, or a male <> female adapter
Just to reiterate the obvious: TWO devices shall be terminated, and the number of devices terminated shall be ONLY two - no more, no less (five is way out, etc).
1) If you have only internal devices this should be:
a) The last device on you internal chain
and
b) EITHER the motherboard terminator packs OR a 25-pin terminator on the external SCSI port.
2) If you have only external devices (weird, but possible) this should be:
a) The last device on you internal chain
and
b) EITHER the motherboard terminator packs OR a 50-pin terminator on a cable attached to the internal SCSI port.
3) If you have internal AND external devices this should be:
a) The last device on you internal chain
and
b) The last device on your external scsi chain.
In any of these configurations any or all devices on the chain can supply additional TERMPWR if required.
EDIT: edited several times for clarity, to repair typos, and to answer all questions in order :-P