HP Surestore 660ex - Optical Jukebox, User Manual

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B-8 Appendix B
Operating This Jukebox on a SCSI Bus
The SCSI Bus and This Jukebox
Adding Devices to the Bus
When considering adding other devices to the jukebox bus, you should consider the
following:
• Will adding a device onto a bus with this jukebox interfere or degrade the
performance of the jukebox or the device that is added?
• If the decision is made to add other devices to the jukebox bus, what SCSI
requirements must be observed to make sure the bus will work?
Before addressing some questions involved in deciding whether to place another
device on the jukebox bus, we can say that, as a general rule, HP recommends that
this jukebox be the only device on a bus. The recommendation is the same whether
the bus is single-ended or differential. One reason for the recommendation is that
interactions on a SCSI bus can be complicated. The more devices on the bus, the
more chance for problems. Another reason is that this jukebox, when fully utilized,
is capable of using up the full bandwidth of the bus. Performance of one or all of the
devices on the bus can be affected.
Consider the quantity of data that the bus will have to carry, the frequency of data
transfer, and the priority of this data transfer in your business. If you are using the
jukebox as a backup or archive device, the demands on the jukebox will be different
than if the jukebox is used for near-online storage where requests for data are more
random in both frequency and amount. If you expect to be using the jukebox at the
same time as the other device, or devices, you should expect to experience a drop in
performance.
Mixing Wide and Narrow Devices
As a preliminary note to this discussion, HP highly recommends that you do NOT
mix wide and narrow buses.
Connecting devices of the same bus width, such as all narrow (50-pin) is a simple
process of daisy-chaining the devices, and terminating both ends of the bus.
However, mixing narrow and wide devices invites problems.
This method should only be implemented by an experienced systems integrator who
is highly knowledgeable about SCSI. Since the wide (68-pin) buses need more data
lines on the bus for their data transfer, it's necessary that the cables connecting the
devices are 68 pin. The data would be lost if the devices were set up as depicted in
the figure below.