Termination issues

Paul D. Aloisi 603-429-8687 aloisi@unitrode.com ALOISI at uicc.com
Fri Nov 15 04:34:30 PST 1996

* From the SCSI Reflector (scsi at symbios.com), posted by:
* "Paul D. Aloisi 603-429-8687 aloisi at unitrode.com" <ALOISI at uicc.com>

CD ROMs generally are low performance devices. A 10X CD ROM is only
about a 1 MegaByte per second transfer rate. Hard Disk are much 
higher performance with transfer rates in the 10 to 40 megabyte
range with large cache buffers. CD-ROMs don't need the high transfer
rates and often are 5 meg - async transfer only, versus hard drives
use synchronous transfer at rates of 10 to 40 megabytes per second
soon to migrate to 80 megabytes per second transfer rates. 

The SCSI bus addressing and control are all done in Async mode - slow
only the data transfers are at the higher speeds.

CD-ROMs often are higher capacitance than the SCSI spec of 25 pF and
often use marginal termination. 

Short buses will run with termination at only one end of the bus. Long
buses must have termination at both ends of the bus. Good active terminators
need to be used, especially when there are several devices on the bus.

Buses will often get standing waves from reflections, the reflection
distance from the end of the bus is often why the 3 and 5th devices
will have problems and the rest will run fine. The SCSI cable is around
90 ohms, when the bus is loaded, the impedance is often in the 60 ohm
range. Several high capacitance drives can actually drop the bus impedance
to the 40 ohm range. The termination is 110 ohms nominal with limited
pull up current. The reflection difference of 40 to 110 ohms is one 
problem, the second is the low impedance means a very small signal that
often relies on the reflected wave for switching. Some signals make the
amplitude and don't need the reflected wave depending of the receiver
and the signal on that line versus others need the reflected wave. This
often causes data errors.

Key Points: Good active termination at both ends of the bus only.
	Spacing between devices 8 inches or more prefered to keep the
	impedance above 60 ohms.
	Try to use only devices that are below 25 pF, 15 pF devices
	will really add margin to a system.

Paul Aloisi


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