Q) 3.8 Which is better, SCSI or IDE? [From: ralf@wpi.wpi.edu (Ralph Valentino)] IDE vs SCSI Non-issues: 1) SCSI and IDE devices cost approximately the same for the same features (size, speed, access time). Shop around for good prices. Advantages of IDE: 1) faster response time (low request overhead) 2) hard drive interface is compatible with RLL/MFM/ESDI: any driver for one (including the main system BIOS) will run the other. 3) IDE controllers are considerably cheaper ($150 and up) than SCSI host adapters. 4) Will always be the boot device when mixed with SCSI. Advantages of SCSI: 1) Supports up to 7 devices per host adapter. This saves slots, IRQ's, DMA channels and, as you add deviceds, money. 2) Supports different types of devices simultaneously the same host adapter (hard drives, tape drives, CDROMs, scanners, etc). 3) SCSI devices will work in other systems as well (Mac, Sparc, and countless other workstations and mainframes). If you change platforms in the future, you will still be able to use your SCSI devices. 4) Automatically configures device type, geometry (size), speed and even manufacturer/model number(SCSI-2). No need to look up CMOS settings. 5) Busmastering DMA (available in all but a few cheap SCSI host adapters) decreases amount of CPU time required to do I/O, leaving more time to work on other tasks (in multitasking OS's only). 6) Software portability - drivers are written for the host adapter, not the specific device. That is, if you have a CDROM driver for your host adapter, you can purchase any brand or speed SCSI CDROM drive and it will work in your system. 7) Will coexist with any other type of controller (IDE/RLL/MFM/ESDI) or host adapter (other SCSI cards) without any special tricks. SCSI host adapters do not take up one of the two available hard drive controller port addresses. 8) greater bandwidth utilization (higher throughput) with multiple devices. Supports pending requests, which allows the system to overlap requests to multiple devices so that one device can be seeking while the second is returning data. 9) Ability to "share" devices between machines by connecting them to the same SCSI bus. (note: this is considerably more difficult to do than it sounds). 10) Bridges are available to hook RLL and ESDI drives to your SCSI host adapter. (note: these tend to be prohibitively expensive, though). Warnings: 1) With otherwise equal drives, IDE will perform better in DOS due to low command overhead. SCSI, however, will perform better in multitasking OS's (OS/2, Unix, NT, etc). If you see speed comparisons (benchmarks), make sure you know what OS they were run under. 2) Most benchmarks only test one aspect of your system at a time, not the effect various aspects have on each other. For instance, an IDE drive may get faster throughput but hurt CPU performance during the transfer, so your system may actually run slower. Similar confusions arise when comparing VLB and EISA host adapters. 3) When comparing two systems, keep in mind that CPU, memory, cache, and bus speed/type will all effect disk performance. If someone gets great I/O performance with a particular controller/drive combination on his Pentium, you should not expect your 386SX-25 to get such I/O performance even with the exact same controller/drive combination. 4) Similarly sized or even priced drives may not perform equally, even if they're made by the same manufacturer. If you're going to compare two drives, make sure they have the exact same model number. (IDE drives usually have an 'A' and SCSI drives usually have an 'S' appended to their model number). Q) 3.9 Can MFM/RLL/ESDI/IDE and SCSI coexist? The PC is limited to two drive controllers total. SCSI, however, is a "host adapter" and not a drive controller. To the rest of your system, it appears more like an ethernet card than a drive controller. For this reason, SCSI will always be able to coexist with any type dive controller. The main drawback here is that on most systems, you must boot off a disk on the primary drive controller, if you have one. That means if you have SCSI and IDE in your system, for example, you can not directly boot from the SCSI drive. There are various ways to get around this limitation, including the use of a boot manager. Q) 3.10 What's the difference between SCSI and SCSI-2? Are they compatible? The main difference between SCSI and SCSI-2 are some new minor features that the average person will never notice. Both run at a maximum 5M/s. (note: Fast and Wide SCSI-2 will potentially run at faster rates). All versions of SCSI will work together. On power up, the SCSI host adapter and each device (separately) determine the best command set the speed that each is capable of. For more information on this, refer to the comp.periphs.scsi FAQ. Q) 3.11 How am I suppose to terminate the SCSI bus? Some basic rules on termination: 1. The SCSI bus needs exactly two terminators, never more, never less. 2. Devices on the SCSI bus should form a single chain that can be traced from the device at one end to the device at the other. No 'T's are allowed; stub length should be kept as short as possible. 3. The device at each end of the (physical) SCSI bus must be terminated, all other devices must be unterminated. 4. All unused connectors must be placed _between_ the two terminated devices. 5. The host adapter (controller) is a SCSI device. 6. Host adapters may have both an internal and external connector; these are tied together internally and should be thought of as an "in" and "out" (though direction has no real meaning). If you have only internal or external devices, the host adapter is terminated otherwise it is not. 7. SCSI ID's are logical assignments and have nothing to do with where they go on the SCSI bus or if they should be terminated. 8. Just because your incorrectly terminated system happens to work now, don't count on it continuing to do so. Fix the termination. Examples: internal external internal external T------|-----|------T T------|-------|-----|------|------T drive drive HA cdrom tape unused unused HA drive drive internal external external T------|-----T T------|------T T------T drive drive HA HA tape cdrom HA cdrom "T" = terminator "|" = connector (no terminator) "HA" = Host Adapter Q) 3.12 Can I share SCSI devices between computers? There are two ways to share SCSI devices. The first is removing the device from one SCSI host adapter and placing it on a second. This will always work if the power is off and will usually work with the power on, but for it to be guaranteed to work with the power on, your host adapter must be able to support "hot swaps" - the ability to recover from any errors the removal/addition might cause on the SCSI bus. This ability is most common in RAID systems. The second way to share SCSI devices is by connecting two SCSI busses together. This is theoretically possible, but difficult in practice, especially when disk drives are on the same SCSI chain. There are a number of resource reservation issues which must be resolved in the OS, including disk caching. Don't expect it to 'just work'. Q) 3.13 What is Thermal Recalibration? When the temperature of the hard drive changes, the media expands slightly. In modern drives, the data is so densely packed that this expansion can actually become significant, and if it is not taken into account, data written when the drive is cold may not be able to be read when the drive is warm. To compensate for this, many drives now perform "Thermal Recalibration" every degree C (or so) as the drive warms up and then some longer periodic interval once the drive has reached normal operating temperature. When thermal recalibration takes place, the heads are moved and the drive may sound like you are accessing it. This is perfectly normal. If you're attempting to access the drive when thermal recalibration occurs, you may experience a slight delay. The only time this becomes important is when you're doing real-time operations like recording / playing sound or video. Proper software buffering of the data should be able to hide this from the application, but software seldom does the proper thing on its own. Because of this, a few companies have come out with special drive models for audio/video use which employ special buffering techniques right on the drive. These drives, of course, cost significantly more than their counterparts. Some other drives offer a way to trigger thermal recalibration prematurely (thus resetting the timer), so if your real-time operation is shorter than the recalibration interval, you can use this to assure your operation goes uninterrupted. Disabling or delaying recalibration is dangerous and should be completely avoided. For more information on the thermal recalibration characteristics of a drive, contact the drive manufacturer directly. Q) 3.14 Can I mount my hard drive sideways/upside down? Old hard drives always had specific requirements for mounting while most modern hard drives can be mounted in any orientation. Some modern hard drives still have mounting restrictions; the only way to be sure is to read the documentation that comes with the drive or contact the manufacturer directly and ask. Restrictions may be model specific so be sure you know the exact model number of your drive. A common misconception is that it is always safe to mount the circuit board side up, this is not the case. When in doubt, look it up. Failure to follow the mounting instructions can result in a shortened lifetime.