{"id":270,"date":"2020-08-18T19:23:47","date_gmt":"2020-08-18T20:23:47","guid":{"rendered":"http:\/\/www.linux-tutorial.info\/?page_id=77"},"modified":"2020-08-22T19:26:40","modified_gmt":"2020-08-22T20:26:40","slug":"this-is-the-page-title-toplevel-105","status":"publish","type":"page","link":"http:\/\/www.linux-tutorial.info\/?page_id=270","title":{"rendered":"The Small Computer Systems Interface SCSI"},"content":{"rendered":"\nThe Small Computer Systems Interface SCSI<\/title>\n<question id=\"234\" text=\"What is a SCSI controller card typically called?\" \/>\n<p>\nThe SCSI-Bus is an extension of your existing <glossary>bus<\/glossary>.\n A controller card, called\na <glossary>host<\/glossary>\nadapter, is placed into one of your expansion slots. A ribbon cable that\ncontains both data and control signals then connects the <glossary>host<\/glossary>\nadapter to your\nperipheral devices.\n<\/p>\n<question id=\"236\" text=\"How many devices does SCSI allow? Wide SCSI?\" \/>\n<question id=\"235\" text=\"SCSI adpaters allow you to add more devices that you have slots on the motherboard.\" \/>\n<question id=\"238\" text=\"Which of the following is NOT an advantage of SCSI over other device types?\" \/>\n<p>\nThere are several advantages to having <glossary>SCSI<\/glossary>\nin your system. If you have a\nlimited number of <glossary>bus<\/glossary>\nslots, adding a single <glossary>SCSI<\/glossary>\n<glossary>host<\/glossary>\nadapter enables you to\nadd up to seven more devices by taking up only one slot with older <glossary>SCSI<\/glossary>\nsystems\nand up to 15 devices with Wide-SCSI. <glossary>SCSI<\/glossary>\nhas higher <glossary>throughput<\/glossary>\nthan either IDE\nor <glossary>ESDI<\/glossary>.\n <glossary>SCSI<\/glossary>\nalso supports many more different types of devices.\n<\/p>\n<question id=\"237\" text=\"What is the original SCSI specification commonly referred to as?\" \/>\n<p>\nThere are several different types of <glossary>SCSI<\/glossary>\ndevices. The original SCSI specification is commonly referred to as SCSI-1. The newer\nspecification, SCSI-2, offers increased speed and performance, as well as new\ncommands. Fast SCSI increases <glossary>throughput<\/glossary> to more than 10MB\nper second. Fast-Wide SCSI provides a wider data path and throughput of up to\n40MB per second and up to 15 devices. There there are Ultra-SCSI and\nUltra-Wide-SCSI\n<\/p>\n<p>\nThe last type, SCSI-3, provides the same functionality as Fast-Wide <glossary>SCSI<\/glossary> as well\nas support longer cables and more devices.\n<\/p>\n<p>\nEach <glossary>SCSI<\/glossary>\ndevice has its own controller and can send, receive, and execute SCSI commands.\nAs long as it communicates with the <glossary>host<\/glossary> adapter using\nproper SCSI commands, internal data manipulation is not an issue. In fact, most\nSCSI hard disks have an <glossary>IDE<\/glossary> controller with a SCSI\ninterface built onto them.\n<\/p>\n<p>\nBecause there is a standard set of <glossary>SCSI<\/glossary>\ncommands, new and different kinds of devices can be added to the SCSI family\nwith little trouble. However, <glossary>IDE<\/glossary> and\n<glossary>ESDI<\/glossary> are limited to disk-type devices. Because the SCSI\ncommands need to be “translated” by the device, there is a slight overhead,\nwhich is compensated for by the fact that SCSI devices are intrinsically faster\nthan non-SCSI devices. SCSI devices also have higher data integrity than\nnon-SCSI devices. The SCSI cable consists of 50 pins, half of which are ground.\nBecause every pin has its own ground, it is less prone to interference and\ntherefore it has higher data integrity.\n<\/p>\n<p>\nOn each <glossary>SCSI<\/glossary>\n<glossary>host<\/glossary>\nadapter are two connectors. One connector is at the top of the card (opposite\nthe <glossary>bus<\/glossary> connectors) and is used for internal devices. A\nflat ribbon cable is used to connect each device to the\n<glossary>host adapter<\/glossary>. On internal SCSI devices, only one\nconnector is on the\ndevice itself. Should you have external SCSI devices, there is a second\nconnector on the end of the card (where it attaches to the chassis). Here SCSI\ndevices are “daisy chained” together.\n<\/p>\n<p>\nThe SCSI-Bus must be <em>closed <\/em>to work correctly. By this I mean that\neach end of the <glossary>bus<\/glossary> must be terminated. There is usually a\nset of resistors (or slots for resistors) on each device. The devices at either\nend of the SCSI-Bus must have such resistors. This process is referred to as\nterminating the bus and the resistors are\ncalled <glossary>terminating resistors<\/glossary>.\n<\/p>\n<p>\nIts fine to say that the SCSI-Bus needs to be terminated. However, that\ndoesn’t help your understanding of the issue. As with other kinds of devices,\n<glossary>SCSI<\/glossary> devices react to the commands sent along the cable to\nthem. Unless otherwise impeded, the signals reach the end of the cable and\nbounce back. In such cases, there are two outcomes, both of which are\nundesirable: either the bounced <glossary>signal<\/glossary> interferes with the\nvalid one, or the devices react to a second (unique in its mind) command. By\nplacing a terminator at the end of the <glossary>bus<\/glossary>, the signals\nare “absorbed” and, therefore, don’t bounce back.\n<\/p>\n<p>\nThe following two figures show examples of how the\nSCSI-Bus should be terminated. Note that Figure 0-6 says that it is an example\nof “all external devices.” Keep in mind that the principle is still\nthe same for internal devices. If all the devices are internal, then the host\nadapter would be still be terminated, as would the last device in the chain.\n<\/p>\n<p>\n<img decoding=\"async\" src=\"scsi_ext.png\" width=529 height=274 border=0 usemap=\"#scsi_ext_map\">\n<map name=\"scsi_ext_map\">\n<area shape=\"RECT\" coords=\"118,158,211,229\" href=\"popup#SCSI Host Adaper#Since there are only external devices, the SCSI host adapter must be terminated.\">\n<area shape=\"RECT\" coords=\"317,80,414,151\" href=\"popup#SCSI Termination#Since there are other devices on the SCSI bus, this device should not be terminated.\">\n<area shape=\"RECT\" coords=\"427,81,522,151\" href=\"popup#SCSI Termination#Since this is the last device on the external SCSI bus is must be terminated.\">\n<area shape=\"RECT\" coords=\"42,233,192,267\" href=\"popup#SCSI Host Adpater#The CPU talks to the SCSI host adapter across the system bus.\">\n<area shape=\"RECT\" coords=\"217,159,467,219\" href=\"popup#SCSI Bus#The SCSI host adpater talks with SCSI devices across the SCSI bus.\">\n<area shape=\"RECT\" coords=\"215,77,312,148\" href=\"popup#SCSI Termination#Since there are other devices on the SCSI bus, this device should not be terminated.\">\n<\/map>\n<p>\n<icaption>Image – Example of <glossary>SCSI<\/glossary> Bus with All External Devices (<b>interactive<\/b>)<\/icaption>\n<p>\n<img decoding=\"async\" src=\"scsiinex.png\" width=532 height=292 border=0 usemap=\"#scsiinex._map\" usemap=\"#scsiinex._map\">\n<map name=\"scsiinex._map\">\n<area shape=\"RECT\" coords=\"215,153,324,225\" href=\"popup#SCSI Host Adaper#With both internal and external devices, the SCSI host adapter should not be terminated.\">\n<area shape=\"RECT\" coords=\"106,90,211,185\" href=\"popup#SCSI Termination#Since this is the only device on the internal SCSI bus, it must be terminated.\">\n<area shape=\"RECT\" coords=\"306,80,419,151\" href=\"popup#SCSI Termination#Since there is another device on the SCSI bus, this first device should not be terminated.\">\n<area shape=\"RECT\" coords=\"427,81,522,151\" href=\"popup#SCSI Termination#Since this is the last device on the external SCSI bus is must be terminated.\">\n<area shape=\"RECT\" coords=\"39,225,204,268\" href=\"popup#SCSI Host Adpater#The CPU talks to the SCSI host adapter across the system bus.\">\n<area shape=\"RECT\" coords=\"214,158,474,218\" href=\"popup#SCSI Bus#The SCSI host adpater talks with SCSI devices across the SCSI bus.\">\n<\/map>\n<p>\n<icaption>Example of <glossary>SCSI<\/glossary> Bus with Both External and\nInternal Devices (<b>interactive<\/b>)<\/icaption>\n<p>\nIf you don’t have any external devices (or only have external devices), the\n<glossary>host adapter<\/glossary> is at one end of\nthe <glossary>bus<\/glossary>. Therefore, it too must be\nterminated. Many host adapters today have the ability to be terminated in\nsoftware, so there is no need for <glossary>terminating resistors<\/glossary>\n(also known as resistor packs).\n<\/p>\n<p>\nEach <glossary>SCSI<\/glossary>\ndevice is “identified” by a unique pair of addresses,\nwhich are the controller addresses that are also referred to as the\n<glossary>SCSI<\/glossary> ID.\nThis pair of addresses is usually set by jumpers or dip switches on the device\nitself. Keep in mind that the ID is something that is set on the device itself\nand is <em>not <\/em>related to location on the <glossary>bus<\/glossary>.\n Note that in the figures\nabove, the <glossary>SCSI<\/glossary>\nID of the devices are ordered ID 0, 6, and 5. Also the SCSI\nID is often set using a combination of jumpers with no 1:1 relationship. (That\nis a pair of pins labeled ID 0 through ID 7) Therefore, you should always read\nthe hardware documentation to determine how to set the ID.\n<\/p>\n<p>\nThis sounds pretty obvious, but some people don’t make sure. They make\n<i><em>assumptions<\/em> <\/i>about what they see on the device regarding how the\nID is set and do not fully understand what it means. For example, I have an\nArchive 5150 <glossary>SCSI<\/glossary>\ntape drive. On the back are three jumpers, labeled 0, 1, and\n2. I have had customers call in with similar hardware with their <glossary>SCSI<\/glossary>\ntape drive\nset at 2. After configuring the tape drive and rebooting, they still couldn’t\naccess the tape drive. Nothing else was set at ID 2, so there were no conflicts.\nThe system could access other devices on the SCSI-Bus, so the <glossary>host<\/glossary>\nadapter was\nprobably okay. Different <glossary>SCSI<\/glossary>\ndevices can be plugged into the same spot on the\nSCSI cable, so it wasn’t the cable. The SCSI-Bus was terminated correctly, so\nthat wasn’t the problem.\n<\/p>\n<p>\nRather than simply giving up and saying that it was a hardware problem, I\nsuggested that the customer change the <glossary>SCSI<\/glossary>\nID to 3 or 4 to see if that worked.\nWell, the customer couldn’t, because the jumpers on the back only allowed him to\nchange the <glossary>SCSI<\/glossary>\nID to 0, 1, or 2. It then dawned on me what the problem was: the\njumpers in the back are in binary! To set the ID to 2, the jumper needs to be on\njumper 1, <em>not<\/em> jumper 2. Once the customer switched it to jumper 1 and\nrebooted, all was well. (Note: I helped this customer <em>before<\/em> I bought\nthe Archive tape drive. When I got my drive home and wanted to check the SCSI\nID, I saw only three jumpers. I then did something that would appall most users:\nI read the manual! Sure enough, it explained that the jumpers for the <glossary>SCSI<\/glossary>\nID\nwere <glossary>binary<\/glossary>.\n)\n<\/p>\n<p>\nAn additional problem to this whole <glossary>SCSI<\/glossary>\nID business is that manufacturers are not consistent among each other. Some\nmight label the jumpers (or switches) 0, 1, and 2. Others label them 1, 2, and\n4. Still others label them ID0, ID1, and ID2. I have even seen some with a dial\non them with 8 settings, which makes configuration a lot easier. The key is that\nno matter how they are labeled, the three pins or switches are\n<glossary>binary<\/glossary> and their values are added to give you the SCSI\nID.\n<\/p>\n<p>\nLets look at Figure 0-8, which represents the jumper settings on a\n<glossary>SCSI<\/glossary> device. In the first example, none of the jumpers is\nset, so the SCSI ID is 0. In the second example, the jumper labeled 1 is set.\nThis is 2<sup>1 <\/sup>or 2, so the ID here is 2. In the last example, the\njumpers labeled 2 and 0 are set, which is 2<sup>2 <\/sup>+ 2<sup>0 <\/sup>= 4 + 1,\nor 5.\n<\/p>\n<p>\nOn an AT-Bus, the number of devices added is limited only by the number of\nslots (granted, the AT-Bus is limited in how far away the slot can be from the\n<glossary>CPU<\/glossary> and therefore is limited in the number of slots). On a\nSCSI-Bus, however, there can be only seven devices in addition to the\n<glossary>host<\/glossary> adapter. Whereas devices on the AT-Bus are\ndistinguished by their base addresses, devices on the SCSI-Bus are distinguished\nby their ID number.\n<\/p>\n<p>\nID numbers range from 07 and, unlike base addresses, the higher the ID, the\nhigher the priority. Therefore, the ID of the <glossary>host<\/glossary> adapter\nshould always be a 7. Because it manages all the other devices, it should have\nthe highest priority. On the newer Wide SCSI-Buses, there can be up to 15\ndevices, plus the <glossary>host adapter<\/glossary>, with\n<glossary>SCSI<\/glossary> IDs ranging from 0 to 15.\n<\/p>\n<p>\nNow back to our story…\n<\/p>\n<p>\nThe device <glossary>address<\/glossary>\nis known as the logical unit number (LUN). On devices with embedded controllers,\nsuch has hard disks, the <glossary>LUN<\/glossary> is always 0. All the\n<glossary>SCSI<\/glossary> devices directly supported by Linux have embedded\ncontrollers. Therefore, you are not likely to see devices set at LUNs other than\n0.\n<\/p>\n<p>\nIn theory, a single-channel <glossary>SCSI<\/glossary>\n<glossary>host<\/glossary>\nadapter can support 56 devices. Devices called <glossary>bridge<\/glossary>\nadapters connect devices without embedded controllers to the SCSI-Bus. Devices\nattached to the bridge adapter have LUNs between 0 and 7. If there are seven\nbridge adapters, each with eight LUNs (relating to eight devices), 56 total\ndevices are therefore possible.\n<\/p>\n<p>\nThe original SCSI-1 spec only defined the connection to hard disks. The\nSCSI-2 spec has extended this connection to such devices as CD-ROMS, tape\ndrives, scanners, and printers. Provided these devices all adhere to the SCSI-2\nstandard, they can be mixed and matched even with older SCSI-1 hard disks.\n<\/p>\n<p>\nOne common problem with external <glossary>SCSI<\/glossary>\ndevices is that the power supply is external as well. If you are booting your\nsystem with the power to that external device turned off, once the\n<glossary>kernel<\/glossary> gets past the initialization routines for that\ndevice (the hardware screen), it can no longer recognize that device. The only\nsolution is to reboot. To prevent this problem, it is a good idea to keep all\nyour SCSI devices internally. (This doesn’t help for scanners and printer, but\nbecause Linux doesn’t yet have drivers for them, it’s a moot point.)\n<\/p>\n<p>\nAlthough the number of <glossary>host<\/glossary>\nadapter manufacturers has steadily decreased in the past few years, Adaptec, the\npremier name in host adapters, has bought up both Trantor and Future Domain.\nAdaptecs biggest competitor for years, Buslogic, was no longer able to compete\nand was taken over by Mylex (a motherboard manufacturer, among other things).\nDespite the decrease in number of manufacturers, the number of models is still\noverwhelming.\n<\/p>\n<p>\nMost <glossary>host<\/glossary>\nadapter manufacturers provide more than just a single model. Many provide models\nfor the entire spectrum of buses and <glossary>SCSI<\/glossary> types.\n<glossary>ISA<\/glossary>, <glossary>EISA<\/glossary>,\n <glossary>PCI<\/glossary>,\n Fast SCSI, Wide SCSI, and Ultra-Wide SCSI are part of the alphabet soup of SCSI\ndevices. You can connect Wide SCSI disks onto a Fast SCSI adapter, although it\nwill still only get 8 bits instead of the Wide SCSIs 16 bits, so it therefore\nonly gets 10Mbytes per second compared to 20Mbytes per second of Wide SCSI.\n<\/p>\n<p>\nUltra <glossary>SCSI<\/glossary>\ndisks can also be connected with the same limitations (it is an 8-bit bus). It\ncan also handle Ultra-Wide SCSI and get 40Mbps. This is not too big of an issue,\nas most of the devices available today can only handle 10Mbps.\n<\/p>\n<p>\nWhen looking at the performance of a <glossary>SCSI<\/glossary>\ndevice, you need to be careful of the manufacturers test results. They can be\ndeceiving. If a test reads 200MB from the disk in 10 seconds, you get an average\nof 20MB per second. What if those 100MB are all from the same track? The disk\nhardware reads the track and keeps it in its own <glossary>cache<\/glossary>.\nWhen the <glossary>host<\/glossary> adapter requests a new block from that track,\nthe hard disk doesn’t need to find the block on the disk,it delivers it from the\ncache. This decreases the access time and increases the <i>apparent<\/i> transfer\nrate of the drive dramatically. The manufacturer can say, in all honesty, that\nthe <glossary>host adapter<\/glossary> has a transfer rate of 20Mbps, though the\ndrive can only do half of this at most. Again, the chain is only as strong as\nits weakest link.\n<\/p>\n<p>\nThis does not mean that Wide <glossary>SCSI<\/glossary>\nor Ultra SCSI are only useful for the companies marketing departments. SCSI has\nthe advantage of being able to talk to multiple devices. For example, it can\nrequest data from one drive and, rather than waiting for the data, free the\nSCSI-Bus (disconnect). When the drive (or other device) is ready, it requests\nthe <glossary>bus<\/glossary> again (reconnect) and the data is transferred.\nWhile the drive searches for the data, the <glossary>host<\/glossary> adapter can\nrequest data from another device. While this device is looking for the data, the\nfirst device can transfer the data to the <glossary>host adapter<\/glossary>.\nBeing able to read or write devices like this means that a host adapter could\nget a <i>sustained<\/i> transfer rate of more that what <i>individual<\/i> devices\ncan handle. (Note that both the host adapter and device must support\ndisconnect\/reconnect.)<\/p>\n<p>\nWide <glossary>SCSI<\/glossary>\ngets its performance gain by the fact it is wide (16 bits versus 8 bits). Ultra\nSCSI, on the other hand, gets the increase through a shorter cycle time. This is\nan important aspect because this makes for a steeper edge on the\n<glossary>signal<\/glossary> (the time from a low to high signal is much shorter,\nand vice versa). This means that the SCSI-Bus has higher requirements regarding\nthe cabling.\n<\/p>\n<p>\nInternal devices usually are connected by flat cable ribbons and present few\nnew problems with Fast <glossary>SCSI<\/glossary>. The maximum length of the\ncable is half of what it could be with older SCSI devices and you must follow\nthe specs exactly. Round cables for external devices have to be created\nspecifically for Ultra SCSI and are therefore more expensive. Although the\nactual data transfer rate between the <glossary>host<\/glossary> adapter and the\ndevice is only as high as the device can handle, the steepness of the edges is\nthe same. This means that if you connect Fast SCSI devices to Ultra SCSI host\nadapters, you still need the special Ultra SCSI cables.\n<\/p>\n<p>\nAnother consideration is that Ultra <glossary>SCSI<\/glossary>\nrequires <i>active<\/i> termination. On the <glossary>host<\/glossary>\nadapter side, this isn’t a problem because the host adapters are designed to\ngive active termination. However, many older devices support only passive\ntermination and therefore cant work on Ultra SCSI host adapters. This really\ncomes into play when larger amounts of data are being transferred.\n<\/p>\n<p>\nPCI devices can generally behave as either masters or slaves. For slave\ndevices, the <glossary>CPU<\/glossary> is responsible for all the activity. This\nis a disadvantage for slave devices because the CPU is often busy transferring\ndata and issuing commands instead of doing other work. This is really an issue\nin multitasking operating systems (like Linux) that have “better” things to do.\nMaster devices, on the other hand, have an advantage here. The CPU only needs to\ntell them where to transfer the data, and they do the work themselves.\n<\/p>\n<p>\nRegardless of whether a device acts as a master or slave, it will take up an\n<glossary>interrupt<\/glossary> line. Single function devices, such as\n<glossary>host<\/glossary> adapters, are given the interrupt INT-A. This means\nthat the actual <glossary>IRQ<\/glossary> (between 5 and 15) will be determined\nby the system <glossary>BIOS<\/glossary>.\n<\/p>\n<p>\nGenerally, you can say that a higher <glossary>throughput<\/glossary>\nis required on a file server\nas compared to a workstation. Although there are applications like CAD or video\nprocessing, which require more <glossary>throughput<\/glossary>\non a workstation than other kinds of\napplications, the major of the work is done by the server. Therefore, it is\nextremely important to consider the performance of your hard disk and similar\ndevices on the server.\n<p>\nDespite reaching comparable prices and sizes, ATA\nharddisks as suited for work in a server because they do not have the throughput\nof <glossary>SCSI<\/glossary>.\n As we discussed previously, SCSI has the advantage of being able to\nhave two devices communicate with each other directly with the need to go\nthrough the <glossary>CPU<\/glossary>.\n In addition, while waiting for one device to find the data, it\ncan “disconnect” itself and you can make a request of another device on the same\nSCSI <glossary>bus<\/glossary>.\n This is especially important on servers as they usually have multiple\ndisks, as well as other kinds of devices.\n<p>\nHere again, the chain to your\nharddisk is only as weak as the weakest link. Therefore, you need to have a SCSI\nhost adapter that can keep up with the hard disk.\n<p>\nLet’s take my Adaptec\n2940U2W <glossary>host<\/glossary>\nadapter which I have in my primary workstation as an example. This\nis an Ultra2 <glossary>SCSI<\/glossary>\ndevice, which gives me a maximum transfer rate of 80\nMbyte\/second. One neat aspect of this <glossary>host<\/glossary>\nadapter, is that it uses a technology\nAdaptec calls “SpeedFlex.”. Internally, there are three connectors. One for\nUltra2 <glossary>SCSI<\/glossary>\n(80Mb\/s), one for Wide Ultra SCSI (40Mb\/s) and one for Ultra SCSI\n(20Mb\/s). Externally, you have two connectors. One 68-pin for Ultra2\n<glossary>SCSI<\/glossary> and\none 50-pin for Ultra <glossary>SCSI<\/glossary>.\n Therefore, you get the maximum <glossary>throughput<\/glossary>\nno matter\nwhat kind of device is connect to the <glossary>host<\/glossary>\nadapter. In addition, one aspect of\nthe SpeedFlex technology is that it can operate the different buses at the\ndifferent speeds simultaneously, so there no performance lost on one\n<glossary>bus<\/glossary> because\nof a slow device on another <glossary>bus<\/glossary>.\n<p>\nThe “W” at the end of the <glossary>host<\/glossary>\nadapter name\nmeans that it has the Wide Ultra <glossary>SCSI<\/glossary>\nconnector. Adaptec also produces the\n2940U2, which has the same specifications, but without the internal Wide Ultra\nSCSI connector and without the external Ultra2 connector. Note that in each\ncase, devices supporting older <glossary>SCSI<\/glossary>\nstandards can still be connected.\n<p>\nIn one one of my machines,\nI have an Adaptec 3940U2, which is also an Ultra2 <glossary>SCSI<\/glossary>\nadapter. One\nkey difference is that this adapter is twin-channel. That means I have two SCSI\nbuses running off of the same <glossary>host<\/glossary>\nadapter. This is extremely useful when you\nhave more device that will fit on a single <glossary>bus<\/glossary>,\n plus added speed. Both of this\naspects make this the perfect adapter for a Linux server (although I use it in\nmy Workstation). Another important difference is that the Adaptec 3940U2\nsupports 64-bit <glossary>PCI<\/glossary>,\n although it is still capable with 32-bit PCI.\n<p>\nOne thing to note is that all of the devices support\nup to 15 devices (plus the <glossary>host<\/glossary>\nadapter per channel). This means that the\ntwin-channel 3940U2 can connect up to 30 devices. In addition, all devices\nsupport cables up to 12 meters long.\n<p>\nOn one machine I have an Adaptec 1542 CF\nhost adapter. The Adaptec 1540 a SCSI-1 and SCSI-2 compliant <glossary>ISA<\/glossary>\n<glossary>host<\/glossary>\nadapter,\nwhich has an 8-bit data <glossary>bus<\/glossary>\nand a maximum transfer rate of 10 Mbyte\/sec. This is\na perfect <glossary>host<\/glossary>\nadapter for workstations which require less performance.\n<p>\nInitially, I wanted to say “low end”, but that tends to create false\nimpressions of something of lesser quality. This definitely does not apply to\nthe Adaptec 1540 family (or any of the Adaptec products for that matter). A few\nmonths ago, I replaced the 1542CF I had after six years because the built-in\nfloppy controller was no longer working correctly. The <glossary>host<\/glossary>\nadapter was working\nfine. I just couldn’t access my floppy drive.\n<p>\nA few weeks later, I put it\nback into the machine as it as the only one with the external connector my\nCD-ROM changer had. Rather than buying an adapter for the connector, I put in\nmy old Adaptec 1542 CF and it has run perfectly ever since.\n<p>\nAll of the\nAdaptec <glossary>host<\/glossary>\nadapters support what is called “scatter gather.” Here, requests\nfor data on the harddisk which are “scattered” all over the drive are “gathered”\ntogether, in order that they be more efficiently process.\n<p>\nThis is similar to\nway an elevator works. Image that four people get into an elevator. The first\none presses the button for floor 12, the next one for floor 16 the next one for\nfloor 3, and the last one wants floor 8. Although the person wanting to go to\nfloor 12 was there first, the elevator stops at floor 3 first, then floor 8\nbefore continuing to floor 12. This is much more efficient than going to each\nfloor in the order the buttons were pressed.\n<p>\nAccessing a hard disk is\nsimilar. On active systems, there will be several requests waiting to\nprocessed. Adaptec <glossary>host<\/glossary>\nadapters will sort the requests based on their physical\nlocation on the hard disk. Interestingly enough, Adaptec refers to this as an\n“elevator sort.”\n","protected":false},"excerpt":{"rendered":"<p>The Small Computer Systems Interface SCSI The SCSI-Bus is an extension of your existing bus. A controller card, called a host adapter, is placed into one of your expansion slots. A ribbon cable that contains both data and control signals … <a href=\"http:\/\/www.linux-tutorial.info\/?page_id=270\">Continue reading <span class=\"meta-nav\">→<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-270","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/pages\/270","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=270"}],"version-history":[{"count":1,"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/pages\/270\/revisions"}],"predecessor-version":[{"id":757,"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/pages\/270\/revisions\/757"}],"wp:attachment":[{"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=270"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}