Termination Needed?

The SCSI Controller on this Mac does not require additional internal termination if no internal drives are in place. As far as I know, only Mac IIfx (and perhaps the Mac IIci) requires an internal device or special terminator when no internal device is attached.

The SCSI bus is a metallic pathway end-to-end. Normal devices do not re-drive the bus, so a terminator placed at the end of the external cable does everything needed for the external bus, and the controller (sitting in the middle) takes care of itself automatically.

Now about the fun project you are talking about. You can certainly install that drive internally, but you will need to provide termination at the end of the internal segment of the bus, regardless of what you have done externally.

Your drive may have two straps: One is called TERM POWER. This allows the drive to supply +5 Volt power (usually through a diode) onto the pin in the SCSI cable reserved for it. Wherever the terminator(s) are on the cable, they can tap into this power pin and use its power to run the terminators.

The second jumper you will see on some drives is something like Termination Enable. This allows the drive to use its on-board termination resistors or similar active termination to actually perform the termination. Older drives, typically only those under 0.5 Gigabyte, use three resistor packs in sockets immediately adjacent to the SCSI connector to provide termination.

An unterminated Bus usually hangs the entire computer, especially under Mac OS X, which is MUCH less forgiving than Mac OS 9. A drive that will not reveal BOTH its Make & Model and its Capacity/Size to utilities like Apple System Profiler and Drive Setup or Disk Utility, cannot be formatted or repaired by any software means -- in its current condition. If its cables and power are good, it has died.

Grant
A small digression. I recently added a JackHammer SCSI card to one of my IIci Macs. I transferred the logic cable from the existing IBM 4.4GB HDD to the JH card's SCSI 2 port, but left the Mac's internal bus unterminated. There was no ensuing problem from that arrangement. However, because I believed it to be notionally preferable, I then used a terminator on the internal bus, with no consequent change in behaviour.

I have no experience with the IIfx, but I am quite prepared to take on faith Apple's requirement for that machine to be terminated internally. After all, they made it. However, the IIci seems to be quite indifferent to the presence or absence of termination on the otherwise vacant internal bus.

Hi Dennis-

The Mac IIfx was the Mac in which Apple discovered that "noise" (caused by the switching of all the signals simultaneously) could occasionally get picked up by the new, fast controller chip and interpreted as a strobe.

To reduce the problem, they implemented the "black terminator" purported too have extra capacitors on the strobe lines to smooth out glitches. In the kit to upgrade my Mac II to a IIfx, they included an internal 50-pin ribbon cable doodad, claiming it should be applied to the internal controller connector when no internal device was being used. I am not sure this doodad was a terminator. It may have only had the capacitor needed to "de-glitch" the strobe lines.

That was the source of the qualification in my statement above.