Frequently Asked Questions - Definition:
Disconnect
What is disconnect?
Disconnect is a special feature of the SCSI protocol.
The SCSI bus is a bus system on which only one device can communicate with another
device at a moment (usually the host adapter commuincates with a SCSI device
- theoretically devices could communicate directly with each other, but usually
this is not used). So if several devices are attached to the SCSI bus, they have
to "share" the bus, i. e. as long as the computer commuincates with the device,
the bus is busy and the other devices can not be accessed.
A read or write command will keep the SCSI bus busy quite some time: The command
is send to the device, the device reads or writes the requested sector and then
returns the data. While the device is searching/ reading/ writing the data the
bus is unnecessarily kept busy.
So if "Disconnect" is activated the SCSI bus will release the bus until it has
processed the command (e. g. read the data into the cache). Only now the device
will inform the controller and transmit the result of the operation.
Therefore "Disconnect" is especially absolutely necessary when doing on-the-fly
copies (if CD-ROM and CD-Writer are attached to the same controller), or else
they will mutually lock the SCSI bus.
How do I activate disconnect?
At first "Disconnect" has to be activated in the settings of the host adapter,
e. g. with an Adaptec® host adapter this is done in the adapter's BIOS. Most
other host adapters use the BIOS or configuration programs as well, you will
find further information in the manual of you host adapter.
Under Windows® 95/ 98 "Disconnect" can be activated/ deactivated for every
single device in the device manager:
"Start" -> "Preferences" -> "Control Panel". Double-click the "System"
applet and select the "Device Manager" tab. Double-click "CD-ROM" and select
the CD-Writer, click on "Properties" and select "Settings". Activate "Disconnect"
here! (While you're at work: Activate DMA as well)
"Disconnect" should be activated with SCSI hard disk, too. For this, open "Drives"
in the Device Manager, select the desired hard disk and activate "Disconnect"
under "Properties" -> "Settings" as well!
Disconnect and the IDE bus
Unfortunately the IDE bus does not support disconnect - activating the switch
"Disconnect" will show no effect.
For this reason a CD-ROM or CD-Writer will lock on certain events (e. g. writing
lead-in/ lead-out, ejecting the medium, automatic repetition on read errors)
another possible parallel attached device - if the parallel attached device is
the system hard disk, thus will lock the complete sytem!
Frequently Asked Questions - Definitions
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