I think you've mostly got it right, but I would not assume that adapters lack termination simply because it is not mentioned. Mind you, I wouldn't buy one from a seller unless it is mentioned.
The thing to understand about shopping for SCSI stuff is that most of the sellers don't seem to understand or think about what they are selling. The number of sellers of SCSI drives on Ebay who don't bother to mention whether a drive has 50 pin, 68 pin or 80 pin connectors is just astounding--as you discovered to your dismay. Plus there's a joker selling SCSI hard drives in the Mac Ebay sections advertising them as 50 pin drives when they are really 80 pin drives with adapters attached. The clue is that if you read his whole item description, you'll notice that he says he can sell the same drive with a 68 pin adapter instead of 50 pin. Oh, and as much as I detest that seller's practices, I think he sells the adapters too, so you might hunt him down and check.
Anyway, so shopping for this stuff is a pain, and there's not much any of us can do to help you. You must either email the sellers with your specific questions and hope that they respond in a meaningful, honest and accurate manner, or find a local store where you can buy them. if you have a Frys or an Altex Electronics locally, I'd recommend that you try there.
As far as understanding termination goes, it is fairly simple in the broad outlines but gets complex in the details. At it's simplest, the rule about SCSI chains is that both ends must be terminated and nothing else must be terminated.
Termination is a configuration of resistors (and IIRC capacitors) on the wires of the cable, but in practice there are four possible sources of termination. There is internal termination in SCSI devices such as hard drives. This is turned on by installing a jumper or off by removing the jumper. There is termination built into the SCSI controller circuitry on later Mac motherboards. The PCI Macs definitely have it. There is SCSI termination built into various SCSI cards such as PCI and NuBus cards. And there are external SCSI termination blocks which you attach to the SCSI cable or plug into a spare SCSI connector on an external SCSI box. Oh, and I suppose a fifth would be termination built into adapters such as you are looking for.
The system of cabling connected to a single SCSI host or controller must form a linear chain. In this case the SCSI host is your macintosh motherboard, but it could be a PCI card if you had a SCSI card. The host need not be at the end of the chain; it could be in the middle if it has connectors for two SCSI cables. In your case, you're only talking about internal devices, so the host will be at one end.
If you are only using internal SCSI devices, then this is pretty simple. Your SCSI chain will look something like this:
T:MB=======HD=========HD=========HD:T
Where MB is motherboard, HD is hard drive, ======== is SCSI cable and :T is a terminator associated with the neighboring device. So in the case above, the left hand termination is on the motheboard and the right hand is on the last hard drive on the cable.
Now 50 pin vs. 68 pin, narrow vs. wide makes things more complicated. Let's use ------ for narrow cable and ==== for wide cable.
What you really have is:
T:MB----------------=HD=------------=HD=-----------=HD:T
Where the little bit of = around the hard drives is there to represent the fact that 80 pin SCA drives are really wide devices.
Now here's the complicating factor. There is termination for the narrow portion of the SCSI cable, but there is also a small block of termination associated with the wide portion of the SCSI cable. So, in the diagram above, the wide portion of the middle two hard drives has no termination. The wide portion is also not used, so you wouldn't think it was an issue, but it is.
So here is what you need:
T:MB--------------=HD1:HBT=---------=HD2:HBT=----------=HD3:T
Where HBT is High Byte Termination. High Byte Termination is termination of only the high byte of the cabling. You do not want to terminate the lower 50 wires, because then you would have termination in the middle of the SCSI chain and that is bad. Termination should exist only at the two ends, except in this special case, where the wide nubs are connected to nothing and so form their own little isolated high byte SCSI chain which needs termination.
So, you need a terminator on the adapter on HD3 which will terminate all 68 pins of the cabling. High and low byte termination. That's just normal 68 pin termination. But you need adapters on HD1 and HD2 which are capable of just terminating the high byte without terminating the low byte.
The motherboard end configures itself automagically, so you don't have to worry about that for this configuration.
Alternatively, you can do this kind of thing:
T:MB-----=HD1:HBT======HD2=======HD3:T
Where you have wide cable between all the hard drives, so you just terminate the high byte at the two ends at HD1 and HD3. Of course at HD3 you also have to terminate the lower 50 pins because they end there too.
The problem with that last configuration is that there really isn't any cabling in the world that will let you do it internally. That is a more practicle arrangement for external SCSI chains, where you have separate cables between each device (instead of one continuous cable with many connectors) and you can just switch from narrow to wide on the back of one of the devices.
Jeff Walther
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