As said before, SCSI devices are smart. The idea is to put the knowledge about intimate hardware details onto the SCSI device itself. In this way, the host system does not have to worry about things like how many heads are hard disks has, or how many tracks there are on a specific tape device. If you are curious, the standard specifies commands with which you can query your devices on their hardware particulars.
The advantage of intelligent devices is obvious: the device drivers on the host can be made in a much more generic fashion, there is no longer a need to change (and qualify!) drivers for every odd new device that is introduced.
For cabling and connectors there is a golden rule: get good stuff. With bus speeds going up all the time you will save yourself a lot of grief by using good material.
So, gold plated connectors, shielded cabling, sturdy connector hoods with strain reliefs etc are the way to go. Second golden rule: don't use cables longer than necessary. I once spent 3 days hunting down a problem with a flaky machine only to discover that shortening the SCSI bus with 1 meter solved the problem. And the original bus length was well within the SCSI specification.
From an electrical point of view, there are two incompatible bus types: single-ended and differential. This means that there are two different main groups of SCSI devices and controllers, which cannot be mixed on the same bus. It is possible however to use special converter hardware to transform a single-ended bus into a differential one (and vice versa). The differences between the bus types are explained in the next sections.
In lots of SCSI related documentation there is a sort of jargon in use to abbreviate the different bus types. A small list:
With a minor amount of imagination one can usually imagine what is meant.
Wide is a bit ambiguous, it can indicate 16 or 32 bit buses. As far as I know, the 32 bit variant is not (yet) in use, so wide normally means 16 bit.
Fast means that the timing on the bus is somewhat different, so that on a narrow (8 bit) bus 10 Mbytes/sec are possible instead of 5 Mbytes/sec for 'slow' SCSI. More on this later.
It should be noted that the data lines > 8 are only used for data transfers and device addressing. The transfers of commands and status messages etc are only performed on the lowest 8 data lines. The standard allows narrow devices to operate on a wide bus. The usable bus width is negotiated between the devices. You have to watch your device addressing closely when mixing wide and narrow.
A single-ended SCSI bus uses signals that are either 5 Volts or 0 Volts (indeed, TTL levels) and are relative to a COMMON ground reference. A singled ended 8 bit SCSI bus has approximately 25 ground lines, who are all tied to a single `rail' on all devices. A standard single ended bus has a maximum length of 6 meters. If the same bus is used with fast-SCSI devices, the maximum length allowed drops to 3 meters. Fast-SCSI means that instead of 5Mbytes/sec the bus allows 10Mbytes/sec transfers.
Please note that this means that if some devices on your bus use 'fast' to communicate your bus must adhere to the length restrictions for fast buses!
It is obvious that with the newer fast-SCSI devices the bus length can become a real bottleneck. This is why the differential SCSI bus was introduced in the SCSI-2 standard.
For connector pinning and connector types please refer to the SCSI-2 standard (see Further reading ) itself, connectors etc are listed there in painstaking detail.
Beware of devices using non-standard cabling. For instance Apple uses a 25pin D-type connecter (like the one on serial ports and parallel printers). Considering that the official SCSI bus needs 50 pins you can imagine the use of this connector needs some 'creative cabling'. The reduction of the number of ground wires they used is a bad idea, you better stick to 50 pins cabling in accordance with the SCSI standard.
A differential SCSI bus has a maximum length of 25 meters. Quite a difference from the 3 meters for a single-ended fast-SCSI bus. The idea behind differential signals is that each bus signal has it's own return wire. So, each signal is carried on a (preferably twisted) pair of wires. The voltage difference between these two wires determines whether the signal is asserted or de-asserted. To a certain extent the voltage difference between ground and the signal wire pair is not relevant (don't try 10 kVolts though..).
It is beyond the scope of this document to explain why this differential idea is so much better. Just accept that electrically seen the use of differential signals gives a much better noise margin. You will normally find differential buses in use for inter-cabinet connections. Because of the lower cost single ended is mostly used for shorter buses like inside cabinets.
There is nothing that stops you from using differential stuff with FreeBSD, as long as you use a controller that has device driver support in FreeBSD. As an example, Adaptec marketed the AH1740 as a single ended board, whereas the AH1744 was differential. The software interface to the host is identical for both.
Terminators in SCSI terminology are resistor networks that are used to get a correct impedance matching. Impedance matching is important to get clean signals on the bus, without reflections or ringing. If you once made a long distance telephone call on a bad line you probably know what reflections are. With 20Mbytes/sec travelling over your SCSI bus, you don't want signals echoing back.
Terminators come in various incarnations, with more or less sophisticated designs. Of course, there are internal and external variants. Almost every SCSI device comes with a number of sockets in which a number of resistor networks can (must be!) installed. If you remove terminators from a device, carefully stock 'm. You will need them when you ever decide to reconfigure your SCSI bus. There is enough variation in even these simple tiny things to make finding the exact replacement a frustrating business. There are also SCSI devices that have a single jumper to enable or disable a built-in terminator. There are special terminators you can stick onto a flat cable bus. Others look like external connectors, so a connector hood without a cable. So, lots of choice as you can see.
There is much debate going on if and when you should switch from simple resistor (passive) terminators to active terminators. Active terminators contain more or less elaborate circuits to give more clean bus signals. The general consensus seems to be that the usefulness of active termination increases when you have long buses and/or fast devices. If you ever have problems with your SCSI buses you might consider trying an active terminator. Try to borrow one first, they reputedly are quite expensive.
Please keep in mind that terminators for differential and single-ended buses are not identical. You should not mix the two variants.
OK, and now where should you install your terminators? This is by far the most misunderstood part of SCSI. And it is by far the simplest.. The rule is: every SCSI bus has 2 (two) terminators, one at each end of the bus. So, two and not one or three or whatever. Do yourself a favour and stick to this rule. It will save you endless grief, because wrong termination has the potential to introduce highly mysterious bugs.
A common pitfall is to have an internal (flat)cable in a machine and also an external cable attached to the controller. It seems almost everybody forgets to remove the terminators from the controller. The terminator must now be on the last external device, and not on the controller! In general, every reconfiguration of a SCSI bus must pay attention to this.
What I did myself is remove all terminators from my SCSI devices and controllers. I own a couple of external terminators, for both the Centronics-type external cabling and for the internal flat cable connectors. This makes reconfiguration much easier.
The terminators discussed in the previous chapter need power to operate properly. On the SCSI bus, a line is dedicated to this purpose. So, simple huh?
Not so. Each device can provide it's own terminator power to the terminator sockets it has on-device. But if you have external terminators, or when the device supplying the terminator power to the SCSI bus line is switched off you are in trouble.
The idea is that initiators (these are devices that initiate actions on the bus, a discussion follows) must supply terminator power. All SCSI devices are allowed (but not required) to supply terminator power.
To allow for switched-off devices on a bus, the terminator power must be supplied to the bus via a diode. This prevents the backflow of current to switched-off devices.
To prevent all kinds of nastiness, the terminator power is usually fused. As you can imagine, fuses might blow. This can, but does not have to, lead to a non functional bus. If multiple devices supply terminator power, a single blown fuse will not put you out of business. A single supplier with a blown fuse certainly will. Clever external terminators sometimes have a LED indication that shows whether terminator power is present.
In newer designs auto-restoring fuses are used who 'reset' themselves after some time.
On modern devices, sometimes integrated terminators are used. These things are special purpose integrated circuits that can be dis/en-abled with a control pin. It is not necessary to physically remove them from a device. You may find them on newer host adapters, sometimes they even are software configurable, using some sort of setup tool. Consult you documentation!
Because the SCSI bus is, ehh, a bus there must be a way to distinguish or address the different devices connected to it.
This is done by means of the SCSI or target ID. Each device has a unique target ID. You can select the ID to which a device must respond using a set of jumpers, or a dip switch, or something similar. Consult the documentation of your device for more information.
Beware of multiple devices configured to use the same ID. Chaos normally reigns in this case.
For an 8 bit bus, a maximum of 8 targets is possible. The maximum is 8 because the selection is done bitwise using the 8 data lines on the bus. For wide this increases to the number of data lines.
The higher the SCSI target ID, the higher the priority the devices has. When it comes to arbitration between devices that want to use the bus at the same time, the device that has the highest SCSI ID will win. This also means that the SCSI host adapter usually uses target ID 7 (for narrow buses).
For a further subdivision, the standard allows for Logical Units or LUNs for short. A single target ID may have multiple LUNs. For example, a tape device including a tape changer may have LUN 0 for the tape device itself, and LUN 1 for the tape changer. In this way, the host system can address each of the parts of the tape unit as desired.
SCSI buses are linear. So, not shaped like Y-junctions, star topologies, cobwebs or whatever else people might want to invent.
You might notice that the terminator issue discussed earlier becomes rather hairy if your bus is not linear..
The electrical characteristics, it's noise margins and ultimately the reliability of it all are tightly related to linear bus rule.
Stick to the linear bus rule!