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- what is mean comp
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- What is a 16550 and do I need one?
  The 16550 is a UART with two 16 byte FIFOs. A UART is the part of a serial port that takes byte-wide (characters) data and converts it to bit-wide (serial) data, and visa versa. The FIFO is a buffer which can hold characters until the CPU is ready to remove it or until the serial line is ready to transmit it. The 'normal' UART in the PC (the 8250 or 16450) only has 1-byte FIFOs. The additional 15 bytes can be useful when the CPU is busy doing other things - if the CPU isn't able to remove data fast enough, it will be lost. The OS or program has to explicitly support 16550 to make full use of its advantages. A very important thing to note is that under DOS, the CPU doesn't have anything else to do, so the 16550 is wasted. Only under multitasking operating systems does it really become useful. The 16550 will *not* make your file transfers any faster, it will only prevent data from being lost and relieve your CPU of some overhead. If you notice system performance dropping like a rock when file transfers are occurring, a 16550 may be helpful. If you see re-transmissions (bad packets) or "FIFO overrun's" during file transfers under a multitasking OS, try the same thing under DOS - if the errors go away, then chances are a 16550 will be useful. If they remain, then your problem is likely to be elsewhere.
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- My floppy drive doesn't work and the light remains on, why?
  If you've played around with the floppy cables at all, chances are you put one of them on backwards. In general, floppy cables aren't keyed to prevent this. Carefully find pin 1 on all floppy drives and the floppy controller and make sure they all line up with pin 1 on the cable. If you have trouble with this, "How do I find pin 1..." elsewhere in this FAQ may be of some help.
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- 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.
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- 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.
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- 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.
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- 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 drive 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.
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- 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.
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- How do I install a second controller?
  The following should solve about 95% (9.5?) of second controller problems, if only to tell you it can't be done! Generic Second Controller Installation: 1) Normally the MFM/IDE/RLL controller is set up as the primary, and the ESDI/SCSI as the secondary; One reason for this is because the ESDI/SCSI controller cards are usually more flexible in their set up and secondly this method seems to work (probably due to reason one). 2) Your primary controller is set up using all the normal defaults: - Floppy at primary address(3F0-3F7). - Hard disk enabled, at primary addresses (1F0-1F7), BIOS address C800 and interrupt 14. 3) Your secondary controller is set up as: - Floppy drives disabled - Hard disk controller enabled, secondary address(170- 177) and interrupt 15. - NOTE: onboard bios set to D400, or D800 can be used, if there is a conflict. 4) Computer BIOS Setup: - Any drive(s) on the primary controller (MFM/IDE), should be entered in the BIOS setup as usual. - You DO NOT enter the drive types for the hard disks on the secondary controller, even if there are only two drives in the entire system i.e., if one drive on each controller you only enter the drive type of the hard disk on the primary controller -- the 2nd drive type is left as not installed (0).
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- Which is better, VLB or ISA IDE?
  If a simple answer is what you want, then yes, in general VLB IDE controllers are better than ISA ones. If you are purchasing or putting together a computer, the relatively small price difference makes the choice for a VLB controller a sensible one. However, if you already have an ISA controller and are wondering whether it's worth upgrading to VLB, it's not that easy. VLB may be faster in principle, the question is if you're going to notice it.
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- Why won't my two IDE drives work together?
  Assuming that the drives are attached to the same controller and they work properly when attached one-at-a-time, you probably don't have them configured properly for Master/Slave operation. When operating 2 IDE drives, one must be designated as "Master" and the other as "Slave." There are jumpers on every IDE drive to configure this. Check your hard drive manuals for the jumper settings for your drives. In general, it doesn't matter which is which - just pick one as master, and make the other slave. In your CMOS configuration, Drive 1 should have the parameters (heads, cylinders, etc.) that match the drive you set as "Master" and Drive 2's parameters should match those of the "slave" drive. In operation, the Master will appear as drive C: and the slave as drive D:. Because not all hard drive manufacturers follow the IDE specifications closely enough, drives from 2 different manufacturers may not work well together. In this case, changing master -> slave and slave -> master (along with the appropriate CMOS changes) may help. If it doesn't, then trying two drives from the SAME manufacturer is the only avenue you have left.
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- Do IDE controllers use DMA?
  No, they do not. This is a rumor that keeps popping up. This may change on the next revision of the standard.
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- Do caching controllers really help?
  The short answer, is that if you are using a multi-tasking operating system with a good memory manager, caching controllers should be ignored. If you are running DOS or Windows, then *maybe* they will help, but I am not sure that they are a good buy. There are lots of people who have said "I put a caching controller in my computer, and it runs faster!". This is probably true, but they never have measured the speed increase compared to putting the same memory into main memory instead. More importantly, the caching controllers cost more money than non caching controllers, so you should be able to add _more_ main memory instead of buying a caching controller.
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