Yamaha A3000, Owner'S Manual

Page: 332 / 376

Connecting external SCSI devices
330
Appendix
SCSI connects a 220-ohm resistance between Vcc and the data lines, and 330-ohms
between the ground and data lines. Termination is achieved when this collective resist-
ance is connected to line 18 of the SCSI bus. Normally, the Vcc signal is output from
line 38 of the SCSI bus to supply the power. If the drive itself has a terminator, the drive
will supply power to the terminator. In this case, it is not necessary to supply Vcc power
to line 38. In general, it is necessary to provide termination at the beginning and the
end of the SCSI bus. This is because termination allows high-speed data transfer unaf-
fected by noise, and keeps the bus signals clean.
■
Realities of terminator installation
Although we have said that a terminator is required at the beginning and end of the
SCSI bus, this is a general principle and not a requirement. For example if the bus is
extremely short (45 cm or less), there are cases in which it is best for there to be a
terminator only on one or the other end. This is something which you should be aware
of when using recent ultra-miniature drives. However if other drives are connected in
addition to these drives, the situation will again be different.
Also, if the SCSI devices are separated by more than 3 meters, it is said that using a
terminator at the 3 meter point is preferable. In this case, three or more terminators
will exist within the SCSI bus.
In this way, the theory and practice of SCSI connections can differ widely. Simply changing
a drive in a system that had been working correctly can cause operation to become
unreliable, or adding a drive to an unreliable system can cause it to start working cor-
rectly. In practice, a certain amount of trial and error is necessary.
■
Reasons for SCSI errors
A SCSI bus operates stably only if all the connected SCSI devices are operating cor-
rectly. If any of the devices are producing noise, the other devices will also produce
errors, and in the worst case, the boot block of a drive can be destroyed. (All data in that
drive will be lost.)
Also, there are cases in which a system on which data appears to have saved may not
have actually saved the data, resulting in data loss.
The following paragraphs will discuss the reasons for such errors, and will provide a
sequence for your troubleshooting.
• Check the SCSI ID
Make sure that there is no conflict between the SCSI ID of each SCSI device includ-
ing the A3000. When shipped from the factory, the A3000’s SCSI ID is set to 6. We
suggest that the internal hard disk normally be set to 4. Be sure to turn off the power
before changing the SCSI ID setting.
• Check the terminator
As discussed above, check the location where the terminator is attached.