SCSI cabling and termination. (small computer systems interface) (Hardware Clinic)
by Mark Minasi
In previous columns, I've discussed choosing SCSI host adapters, setting SCSI ID values, and enabling or disabling SCSI parity.
This month, we'll tackle the last two steps of physically installing a SCSI subsystem: cabling it and setting up its terminators.
Many of you will put only one or two SCSI devices on a PC, but SCSI can easily support seven peripherals off a single SCSI host adapter. (A single SCSI host adapter can actually support tens of thousands of devices--in theory. But I wouldn't try it.)
Multiple devices are attached to a single SCSI host adapter via daisychaining over several kinds of cables: a 50-conductor Centronics connector, a DB25 connector, a miniature DB50 connector, and a 50-pin ribbon cable. A 50-conductor Centronics connector looks like the connector on the printer end of your parallel cable, only larger-- your printer uses a 37-conductor connector. A DB25 connector is the kind that you find on a serial port or on a parallel cable--on the PC end. Miniature DB50s look somewhat like DB25s, but there are 50 small pins or holes in the same space that the DB25 uses for 25 pins or holes. Also, DB50 connectors generally have buttons that you must press to connect or disconnect them.
Some external SCSI devices have two Centronics 50 connectors on the backs of their cases--two connectors so that they can be part of a daisychain. Older SCSI devices-- CD-ROM drives that follow the pre-SCSI-2 standard, mostly-- are the only devices that use the DB25. The miniature DB50 shows up on the backs of some SCSI host adapters and on a few devices. For example, the Hewlett-Packard ScanJet Ilc has a miniature DB50 connector.
You can pretty much ignore the DB25, but you can't ignore the Centronics 50 and the miniature DB50, so there are three possible kinds of SCSI cable you might have to lay your hands on: miniature-DB50-to-miniature-DB50, miniature-DB50-to-Centronics-50, and Centronics-50-to-Centronics-50 cables. And you have to be careful where you get them. To explain that, let me relate a short war story.
I installed a CD-ROM drive on a server so that I could install Windows NT Advanced Server on my LAN. (Windows NT Advanced Server is terrific, by the way; if you're thinking about moving to it, don't hesitate.) The CD-ROM drive used the Centronics 50 connector, so I pulled a standard SCSI cable with a Centronics 50 connector on each end out of my cable pile. I plugged the CD-ROM drive into the SCSI adapter's interface port on the back of my PC, and the problems started. My tape drive and hard disk started acting up. Running Chkdsk revealed lost clusters and invalid subdirectories on my hard disk-- but the messages referred to different parts of the disk every time I ran Chkdsk! The tape drive wasn't recognized about half of the times I booted the system, although it had been working fine for months-- clearly, the new guy on the block (the CD-ROM drive) was causing trouble.
I changed the host adapter to SCSI, giving up SCSI parity in the process and engendering a sense of deep foreboding. Then I turned to installing Windows NT Advanced Server. The README file that came with it said, "The SCSI and CD-ROM support built into Windows NT 3.1 requires that CD-ROMs provide SCSI parity to function properly." Oops.
I started fussing with the SCSI devices to get the CD-ROM drive to support SCSI parity. There was a jumper included on the CD-ROM drive to control whether or not SCSI parity would be used, so why wouldn't it support SCSI parity?
On the off chance that I had a bad cable, I went back to the cable pile to see what else I had. I found another dual Centronics 50 cable identical to the cable that I was using and another dual Centronics 50 cable that was about twice as thick as the first two. I tried swapping the original thin cable for the other thin cable. No difference. But when I used the thicker cable, everything started working! I got full SCSI-2 support, as well as SCSI parity.
A few calls to cable places confirmed that there were two kinds of dual Centronics 50 cables. The thin ones work fine for SCSI but not for SCSI-2. The thick ones are good for both. Look for cables from Amphenol, Quintec, and lcontec, and your cables should work fine under SCSI-2.
External devices, as I've said, tend to have two SCSI connections on them so that they can support the SCSI daisychain. Internal devices, by contrast, use only a single 50-pin header connector. Internal SCSI cables are just 50-conductor ribbon cables, looking somewhat like fatter-than-usual hard disk cables.
Now that you know about cabling, before popping the top back on your PC, there's one more thing that needs doing. You must terminate the SCSI chain.
To terminate is to provide a voltage and resistance on either end of a cable, so that the entire bus has a particular set of electrical characteristics. Without this resistance, the SCSI cables cannot transport data around without significant error rates. (This process will work sometimes, despite what some people claim, but it won't work reliably.) Complicating things a bit, as you'll see, is the fact that there are two kinds of termination, and they aren't really compatible. Passive termination is employed by earlier SCSI devices. Active termination is employed by SCSI-2 devices.
I've said that some SCSI devices are installed internally in the PC and connect to the host adapter with a ribbon cable. Other devices are installed externally and connect to the host adapter with one of the three kinds of common external SCSI cables. There are also internal and external terminators, as well as SCSI devices that have terminators built in. The terminators you'll see include the following.
* An internal SIPP (Single Inline Pin Package) terminator on the host adapter and/or hard disk
* A separate external SCSI terminator (reliable only under SCSI, not SCSI-2)
* A device with built-in termination that's enabled or disabled with a DIP switch or a jumper
* A device with built-in termination that cannot be disabled
An internal SIPP terminator looks like a colored plastic blob with a row of little metal legs sticking out of it. You'll find SIPP-type adapters usually on SCSI hard disks or host adapters. SIPPs often show up on the host adapter itself because it needs termination, and SIPPs don't take up much space. If you have a device that terminates with SIPPs, you'll probably see three of these SIPPs on a host adapter. If you don't need termination on a device equipped with SIPPs, just remove them (gently--you may need to reinstall them one day) by working them out with needlenosed pliers. Once you've done that, put them in an envelope, seal it, label it "SCSI terminators," and put it with your valuables.
Newer host adapters don't terminate with SIPPs, however. Adapters like the Adaptec 1542C can terminate or not with a software command. If this doesn't sound like manna from heaven to you, it's a sure bet that you've never spent hours opening and closing a PC, flipping jumpers, and trying to get everything working. This simplicity is a feature I'd look for in a host adapter.
Older SCSI systems make use of external SCSI terminators, terminators that can attach to one of the Centronics 50 connectors on the back of a SCSI device. External terminators look like Centronics connectors without cables attached to them. They clip onto one of the Centronics connectors on the back of the last external device on your SCSI daisychain. The external Centronics 50 connector terminator may not work in a SCSI-2 environment. You're best off these days looking for SCSI devices that offer active termination. The explanation is coming up.
If you look at the back of an external SCSI device, you may notice a switch labeled Termination. It can be flipped on or off, so if this SCSI device is the last on the chain, all you need to do is flip the switch on. If this device isn't the last device on the chain, flip it off.
You've already read that SCSI uses passive termination, and SCSI-2 uses active termination. What's the difference? Are they compatible?
First, the differences. Passive termination employs two resistors on either end of the SCSI bus. A 220-ohm resistor is tied to the termination voltage (one of the SCSI lines), and a 330-ohm resistor is tied to a ground. Active termination is a more reliable approach that uses a single 110-ohm resistor to the termination line.
Second, what about compatibility? While you may experience different results, most of the SCSI-2 setups I've worked with require active termination. If you use one of the old plugtype terminators, SCSI-2 may not work. Active termination requires electrical power provided by some device (the SCSI drive, the host adapter, or whatever), so there's no simple plug that will provide active termination on a SCSI-2 chain. You must have a device that supports active termination on both ends of a SCSI-2 chain, or you're likely to experience problems.
One more thought before I go: A number of devices offer the ability to convert a simple parallel port into a SCSI device; perhaps the best known is from a company named Trantor. In my experience, these devices will not work unless the SCSI device that you're trying to attach can provide active termination. This represents just one more reason to double-check that the SCSI devices that you buy can provide active termination.
Do you have a tough hardware problem you'd like Mark to tackle? Let him know about it by calling (900) 285-5239 (sponsored by Pure Entertainment, P.O. Box 186, Hollywood, California 90078). The call will cost 95 cents per minute, you must be 18 or older, and you must use a touchtone telephone.