The following is in response to some of the many general questions that arise from time to time on the Adaptec CD-R users list regarding SCSI. The information is not definitive and I claim no responsibility for it being 100% correct but I feel it covers most areas with sufficiently accurate information to make it interesting and therefore worth reading.
AHA2940UW. You must not use all three connectors at the
same time. If you do so you will violate the SCSI spec. by turning your SCSI
bus into a T. Only use two connectors at any one time.
Technical Note. The SCSI Bus is an end to end bus, i.e., it does not
support branches. There is no allowance within the SCSI spec. for SCSI Bus
branches (although there is a stub allowance). Therefore, as the 2940UW and
other SCSI controllers utilize 3 connectors, for connectivity purposes only,
you must not use all three at the same time. All three connectors are
physically wired to each other, no buffering or re-clocking circuitry is
present between the connectors. For the AHA2940U2W and the new AHA2940UW Pro
you can use all three connectors, in fact on the U2W you can use all 4. This is
due to a SCSI extender chip, the AIC3860 fitted to both cards. This little
device creates an isolated extension of the main SCSI bus allowing you to use
the extra connector plus electrically isolate devices between the main bus and
the extended bus.
Technical Note. The SCSI bus is still one logical bus on the U2W and
UW Pro, therefore you still must ensure that each device has its own SCSI ID
and that each bus, primary and extended, is terminated correctly.
Termination. Only terminate the ends of the SCSI bus, not
the middle or only one end but both ends of the bus. Do this either by enabling
terminators on the device or by terminating the cable/external connector.
Technical Note. For single-ended SCSI there are two types of
terminator, Active and Passive. Passive Terminators are to be found on older
SCSI I type peripherals and can be identified as a row of three Single In Line
(SIL) resistor packs, normally stamped with 220/330. This refers to the
resistive value of the packs in Ohms. Due to their nature of operation, passive
terminators should only be used if the speed of the SCSI bus is no greater than
5MB/s. If the speed of the bus is above this as in Fast SCSI then active
terminators should be used. Active terminators give a cleaner signal on the
SCSI bus and are therefore suited for use at higher bus speeds.
SCSI IDs. Try to leave 0 and 1 for disk drives. It is not
strictly necessary to do this but traditionally 0 and 1 have been reserved for
disk drives and it won't do you any harm if you continue this tradition.
Technical Note. SCSI IDs have a priority order; if two devices wish to
gain access to the bus at the same time the device with the highest priority ID
will gain access first. SCSI ID 7 has the highest priority followed by 6, 5, 4,
3, 2, 1 and 0, then by 15, 14, 13, 12, 11, 10, 9 and 8 for Wide SCSI devices.
Wide/Narrow SCSI. You can mix wide and narrow devices on
the same bus but you may need to use special converters/terminators like the
ones from HYPERLINK http://WWW.TMCSCSI.COM
Try to convert from wide to narrow, that way is easier to implement and
the wide devices can run ... well. wide. Keep the SCSI ID of the controller at
ID 7, moving it above ID 7 will result in the narrow devices being unable to
see the controller. Adaptec and other controllers negotiate for Wide SCSI
transfers on a per device basis, the wide part only being used during actual
data transfer.
Technical Note. Address lines are used on the SCSI bus, one for each
ID. If you have a narrow device it will not be equipped with the necessary
address lines to see above ID 7.
Disconnect/Reconnect. This feature is normally enabled; it
allows multiple devices to overlap transfers/commands on the SCSI bus and
therefore maximizes the available SCSI bus bandwidth; its what makes SCSI
suitable for multitasking environments. However, it can also cause a good run
in coaster manufacture. Try disabling it as part of your normal trouble
shooting. This setting can also be set on a per device basis on some SCSI
controllers.
Technical Note. The term disconnect refers to a SCSI device's ability
to disconnect logically from the bus. This is not a physical
disconnect!. Allowing devices to logically disconnect frees up the bus to allow
other devices to gain access and perform data transfers and makes more use of
the available bus bandwidth. Its benefit is appreciated most in multitasking
environments with multiple peripherals attached to the bus.
Synchronous Transfers. If this were a perfect world then
every device ever produced would handle the request for Synchronous Transfers
correctly. However, it isn't and they don't. If you are trouble shooting, then
try disabling this option in the SCSI card's BIOS. CD-ROMs, CD-Rs, Tapes and
Scanners have all been known to handle the controller's request for Sync.
Transfers poorly. Today's hard drives are immune to this but ... well, just
beware. Again this option is enabled on a per device basis so this allows
devices to run at different speeds on the SCSI bus.
Technical Note. Synchronous Transfers are negotiated between devices
just like Wide SCSI transfers. Different rates can be supported on the same bus
at the same time. The negotiation involves a speed parameter defining a minimum
period of time between the leading edge of successive REQ (request) or ACK
(acknowledge) signals plus an offset parameter for the number of REQs received
by a device before it has to respond with an associated ACK. Each REQ-ACK
handshake defines the transfer of one byte or one word of data transfer. All
Data transfers on SCSI can be performed at the full burst rate of the bus if
that rate has been negotiated beforehand between devices, however command,
message, status phases, etc. are performed in asynchronous mode no matter how
fast the burst rate of the bus is.
Ultra SCSI & Ultra2 SCSI. Ultra SCSI is another option
that can be enabled on a per device basis. Apart from increasing the speed of
the bus, it has also increased the number of headaches experienced around the
world by users and Tech. Support people alike! Theory goes that you can mix
Ultra and non-Ultra devices on the same bus. Many people do and it all runs OK
for the lucky ones. Because of the cable limitations imposed by just one device
running at Ultra SCSI speeds, my suggestion if you do have issues is to A)
first make sure your cable is A1 OK; B) make it as short as possible; C)
separate the Ultra devices from the non-Ultra by installing another card.
Ultra2 SCSI, on the other hand, is far easier to implement. It offers another
step forward in SCSI speed to 80MB/s burst and also offers an electrically
different signal scheme called Low Voltage Differential or LVD. This signaling
scheme suffers none of the major issues seen with Ultra SCSI (short cable
length vs number of supported devices, etc.) but it is electrically
incompatible with Ultra SCSI devices in its native form. This becomes a
non-issue as Ultra2 devices employ a scheme where they can detect if they have
been installed on a non-LVD bus (Ultra or FAST) and revert to the maximum speed
and electrical characteristic of that bus thus making them backward compatible
with previous generations of SCSI.
Technical Note. The term Multimode I/O cell is often used when
referring to LVD devices' ability to detect what type of bus they are attached
to. However, since LVD devices are generally not compatible with the SCSI High
Voltage Differential bus, only HVD devices should be used on a HVD bus.
Cables. Not all cables are created equal. Ultra/Wide and
Fast/Wide SCSI cables are different. They have different specifications. Make
sure yours comply with whatever you are running. External cables and connectors
can cause you issues if they are not up to spec. You only get what you pay for!
It is asking too much when you spend $100s on the latest all singing/dancing
PCI controller and expect it to drive a device via a substandard cable.
Technical Note. Ultra2 cables are essentially the same as Ultra
cables. OK, they are not the same! But if you have a stable working Ultra SCSI
system and wish to move to Ultra2, then the cables you currently have will work
in most cases, the only reason to upgrade would be if your system was on the
tolerance edge.
Bus Mastering. Bus-mastering PCI cards greatly ease the
worry/pain of CD-R work. Sure they are not necessary and many people do use ISA
cards and non-mastering controllers, like the early AHA2920 but at the end of
the day they are affordable, like the AHA2910 / 2930 and give you that good
piece of mind feeling.
Technical Note. Bus Mastering or Bus Mastering DMA (Direct Memory
Access) refers to an I/O device's ability to control a data transfer directly
into your computers main memory without CPU intervention. This makes better use
of the available CPU bandwidth by not saturating the CPU with interrupts that a
non-mastering card would generate. Today all PCI SCSI cards feature BM DMA; it
is a specific requirement for the Microsoft PC99 initiative. The early Adaptec
AHA2920 was a re-badged Future Domain PIO (Programmed I/O) Card but all later
Adaptec cards feature BM DMA. ISA cards are to be phased out as part of the
Microsoft PC99 initiative (PC99 Basic Requirements, Chapter 3, page 66 Section
3.28). This means that all new systems sold after 1 January 2000 should be sold
without an ISA slot or slots if the system vendor wishes to use the PC99 logo.
Fixes. If it ain't broke, don't fix it!
If you have anything to add to the above, correct or dispute then please
email me in a constructive manner at nealg@freenetname.co.uk You may
distribute the above as you wish, all I ask is that you acknowledge the Author
!!
Neal Gibbons
E-mail me at cdrecording@mrichter.com
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