Once you have all these devices hooked up, you need to terminate the bus.  Terminate?  Yup, get out the scissors.  No, not that kind of termination.  Termination is needed to seal off the two ends of the bus.  Here's an example of termination.

External devices always give good examples of termination because you have to use plugs that terminate the bus.  External devices are a little different from internals in that they have an In and an Out plug.  One uses lengths of cabling to chain the devices together instead of one long cable with many plugs.  All the terminators really are resistors that absorb the signals from certain wires.  If there wasn't any termination, the signals would actually bounce off the end of the bus and interfere with all sorts of stuff.  This is a Bad Thing®.  So how does the adapter get terminated?  All adapters today have automatic termination.  That is, they can turn their termination on and off depending how things are hooked up to them.  Internal devices are similar but their termination can be turned on and off via a jumper.  This negates the need for resistor packs because all you have to do to terminate your bus is to activate termination in the last device, and you're set.  Here's an example:

Notice how the controller card's termination is off, and the two end device's terminations are on.

Now that we have the basics of termination, that is keeping the two ends of the bus capped off, let's look at some of the more complex issues you might run into.

One of the first stumbling blocks in the termination game is the fact that all LVD (Ultra2) devices don't have that handy internal termination.  This introduces the need for resistor packs for internal stuff, too.  I'm not sure if HVD devices have (had, really) internal termination or not, but since HVD is pretty much dead, who cares?

This brings me to my next topic, the different types of terminators.  There are 5 types of terminators out there:

• Passive
• Active
• Forced Perfect
• LVD
• HVD

Passive termination is the old-skool method of termination.  That is to say, it sucks.  It drains too much power, and isn't good for stuff running over 5MHz.

Active termination is a modern approach that does a much better job of terminating than passive termination.  

Forced Perfect termination is a complex and controversial method that is used in high-speed, complex systems.  Having a wide range of devices over a long cable can introduce extra instabilities, and FPT takes care of these.  The reason it's controversial is that it technically violates some SCSI standards that could cause some chips to fail prematurely.  Note that Passive, Active, and FPT termination are for SE busses only.

LVD termination works similarly to Active termination but it is designed for LVD busses.

As HVD and LVD are fairly similar, the same applies for HVD terminators.

Note that each type of terminator can only be used with the applicable bus type.  For example, I had a 5 connector cable that came with a LVD terminator.  I had my Ultra2 hard drive and Ultra CD-R hooked up to my Ultra Wide adaptor, with the bus terminated with the LVD terminator.  As the bus was in SE mode, the LVD terminator was the wrong one, and I could only detect one of my devices at a time.  Needless to say, I was forced to drop another $20 on an Active terminator; but at least my system worked.

Let's take a look at a few examples that demonstrate issues dealing with terminating Wide and Narrow busses.  Before you get into the ugly table, let me mention the existence of 50 to 68 pin converters.  These let you plug 50 or 68 pin devices into 68 or 50 wire cables.  Not too hard, eh?

This setup uses a controller card that happens to have a 50 pin (Narrow) plug and a 68 pin (Wide) plug.  On the narrow cable there is a narrow drive, and no converters are needed.  Its termination is on, of course.  However, on the 68 pin cable there is a narrow and a wide device.  That's why the 50 to 68 pin converter is needed.  Capping off the end of the bus is the wide drive with its termination on.  What's the deal with the Low and High bytes?  Well earlier I mentioned that a wide bus is 2 bytes wide.  It consists of a Low byte and a High byte.  That wide device on the end will have termination for both the Low and High bytes.

Here's a view from inside the controller, or at least what the controller is doing.  Remember when I said the controller has automatic termination?  It turns out that it can do a lot of things.  It can control its Low and High byte termination separately.  In this case it would turn its Low byte termination off, because both of its plugs are using the Low byte.  It would turn its High byte termination on because it needs to cut it off lest it mess things up.  If you have a manual for your controller, it should list all the different automatic termination settings it will execute.

Let's say you're using a 68 pin cable and on the end of it you have a couple narrow devices.  Let's assume you hadn't read my excellent *cough* page and you were using regular 50 to 68 pin converters.  Where's the High byte going to go?  Never fear, the High-Byte Active Converter is here!  What this converter does is in addition to converting a 68 pin connection to a 50 pin plug, it also terminates the High byte for you!

Here's Adaptec's guide to mixing Wide and Narrow devices.  It pretty much says the same things I've said here, but it gives a bunch of examples.  Here's the link.

Now you have the basics of setting up and getting a SCSI system to work!  Take a look at the SCSI vs. IDE page for just that, the Setups page for a look at some example systems, the Hardware page for the hard stuff of SCSI, and the other few pages I have here.

Hey look, there's a terminator (LVD)