|ENGINEERING TRIPOS PART II A and EIST I|
|EIET LABORATORY||ENGINEERING AREA ACTIVITY|
Areas 5,6,7 and 8
|INVESTIGATION OF THE DESIGN OF CD PLAYERS|
SUBTOPIC: CD-ROM DATASTORES
CD technology, first developed for audio disks, is now the basis of a high-capacity Read-Only disk system: CD-ROM. The disks themselves are similar to audio CDs, although their internal organisation is more complicated and extra Error Control Coding is used, since errors on a computer disk are more serious than errors on audio disks.
In this investigation, you will have available a typical configuration of a PC connected to an external CD-ROM drive using a SCSI (Small Computer Systems Interface) cable. To let you see signals on the SCSI cable, a 'breakout box' is provided, which allows a Logic Analyser to be connected to the cable.
The aim of this investigation is observe and explain the time taken to get data from the CD-ROM, which involves a number of components:
Familiarity with DOS will help you, but is not essential (although it means you'll have to find out about various DOS commands).
* Available to you when you start doing the activity.
To make efficient use of your time, you need to plan your investigation. The following is a suggested outline plan, but you are free to vary it:
Read the handouts and the selected parts of the ANSI standards, which have been photocopied. These include Table 2 of ANSI X3T10, which gives the contact assignments of the SCSI interface in use here. (There are 12 different variants!)
Familiarise yourself with the equipment. The CD-ROM is set up on drive E: and the CD-ROM provided is a 'Radiospares' catalogue. To select the CD-ROM as your main drive, get a directory listing, and type an example file, type:
Drive D: is the RAMDRIVE ie. up to 16Mb of RAM which behaves like a disc drive but is very fast. To get the clearest timing diagrams, it is best to copy to this drive. Why ?
Note that in DOS there is a cacheing system called 'SMARTDRV' which can cause some confusion. For example, if you 'TYPE' a file twice in succession, the PC reads it from the drive the first time, but the second time it knows that it already has the contents of that file stored in RAM, so it doesn't bother to read it again from the drive. We have therefore disabled 'SMARTDRV' in the PC's start-up file. Nevertheless, you should be on the lookout for occasions when software in the PC 'doesn't bother' to read from the actual drive.
Familiarise yourself with the logic analyser, and use it to observe and interpret the transactions on the SCSI bus when you perform simple operations like a directory listing 'DIR' or typing a file. Further notes and details on this will be supplied.
Plan initial investigations and then a scheme of measurements to allow you to characterise the sequence of events in a file transfer and explain it to others.
Attempt to estimate how much time is spent in the various parts of a data transfer mentioned at the start: the SCSI Initiator-Target control/data exchange procedure; moving the CD player read head to the right place; disk rotation to the right place; driver software in the PC.
You can only make estimates, and it may not be possible to separate the various delays inside the CD player. A further complication in the CD player is that it has its own 64Kbyte RAM cache (see manual). Have a look at the manual (section 8) also to find out about the quoted data transfer rates of the device.
For your presentation, make an assessment of (a) the CD-ROM drive as a data store (compared for example to magnetic disks) and (b) the performance of the SCSI bus.
You may like to consider questions such as: how does the CD-ROM compare to other bulk storage media (cost, density, resistance to physical damage, convenience, etc.)?
For a CD-ROM drive, are the main limits due to the SCSI bus or the drive? Do they matter anyway? What developments of SCSI are there?
You may also like to think about the impact of CD-ROM technology as a core technology in 'multimedia': data + sound + still pictures + movies.
Dr P.R. Palmer, October 2002