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Originally Posted by thomas
well, SCSI-2 *is* "normal" SCSI.
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No it isn't. There is no such thing as "normal" SCSI.... However, given the many flavours of interface, bandwidth, and max throughput, its hardly suprising the 3 defined standards are confused!
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There is SCSI-1 which usually is used for external devices like scanners and uses a 25 pin connector which looks like printer cable (but is not).
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No. SCSI is not defined by the number of pins on the interface/cable. SCSI-1 defined a 8bit, 5MHz transfer rate, giving 5mb/sec bandwidth less command overhead and latency.
SCSI defines a whole bunch of things, such as bandwidth, transfer rate, max cable length, signalling methods, etc etc.
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SCSI-2 has 50 pins and is used for internal devices like CD-ROMs, CD-writers and old HDDs.
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SCSI-2 was an improvement to the SCSI-1 standard, adding new commands and improving the efficiency and transfer rate of the bus. There weremany "flavours" of SCSI-II, in the main Fast-SCSI which introduced 10Mhz 8bit for 10mbit; wide-SCSI (5MHz,16bit) and fast-wide SCSI (10MHz, 16bit).
The 50pin bus is restricted to 8bit transfer, and such devices that use the 50pin interace are refered to as 'narrow' devices. Thus, one can have an 'ultra-narrow' deivce that supposedly operates at 20MB/sec; 8bit @ 20Mhz.
Confusingly, the 'wide' 68pin bus is physically narrower than the 'narrow' 50pin bus...
And it gets worse. SCSI-1 and 2 essentially had a max cable lenght of 6m (including the interface circuitry on the device....). HVD - High Voltage Differential - extended this to 25m using higher voltages...
SCSI-3 introduced LVD - low voltage differential - a way of lowering the sgnalling voltage to reduce cross-talk on the bus. However, it uses the same 68pin connector as fast-wide/ultra-wide SCSI devices....
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Nowadays SCSI-3 aka UW-SCSI (Ultra Wide) is used for HDDs which have 68pin connectors, or even U2W-SCSI which is twice as fast AFAIK.
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SCSI3 basically doubled the bandwith. Ultra = 20Mhz. Ultra2 = 40MHz. After U2, *all* connections were wide, and hence there are no "ultra only" devices.
It includes the "obsolete" ultra-wide (20MHz, 16 bit for 40mb/sec) and U2 (40MHz, 16bit for 80mb/sec) devices.
[QUOTE]With the correct adapters all SCSI standards are compatible to each other. Of course you won't run SCSI-3 speed on a SCSI-2 controller.[/UNQUOTE]
yes... and no. Dont ever ever ever try to fit a SCSI-1 device on a SCSI-2 or higher bus. SCSI-2 introduced the concept of device initiation, which can confuse SCSI-1 controllers.
You can attach narrow (8bit) SCSI devices to a wide SCSI bus... but it needs a 50-68pin converter. If you ever do this, it *must* be the last device on the chain, as a proper converter has a high-byte terminator included to prevent the "extraneous" signals going to the narrow device. On a real real adaptor, its a parallel wired component, so the high bytes still pass through on the bus. However, not all are "real real", so why take the risk (ie do you want half the data destined for a wide device to go missing half way down the chain as it hits a terminator?). Secondly, on wide SCSI-2 chains, U2 and some U160 chains, adding a narrow device will slow the whole bus down to that device's speed... nto a good idea. SCSI-3 defined the concept of "domain validation" and "domain isolation", which meant that the adaptor queried each device to determine it's capabillities, then talked to each device accordingly. it took time for HBA manufacturers to include this in their products. best off adding an older SCSI-II card for using narrow SCSI devices.
Dont ever ever stick a non-HVD SCSI device onto an HVD SCSI adaptor - it will be fried.
Whether an LVD device will be fried on an older SCSI adaptor I dont know.... and aint about to find out!
Will a wide device work on a narrow chain? Yes, but there are a lot of things to be careful of, notably hi-bte termination, device ID, and wide negotation.
A really good place for starters is
http://www.datapro.net/techinfo/SCSI...#_Hlk410547597
Bear in mind some of the info is out-of-date, as the FAQ has been going for years and was last updated Sep 2001...
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IMHO your SCSI-2 drive should perfectly fit on the 2091 controller.
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And, by a very long and tortuous route... I agree :-)