Ultra SCSI-2 ??? (You mean SCSI-3, SPI-2)

Skip Jones sk_jones at qlc.com
Tue Oct 3 15:45:59 PDT 1995


So, what happens if a SCSI-1, or SCSI-2 old fashion single-ended device is on
the bus, which of course does not monitor the bus for much of anything, much
less what the level of DIFFSENS is?  As I understand it, this would simply pull
the level of DIFFSENS to ground (thus less than 0.6V), thereby causing the bus
to become single-ended. 



Skip Jones
Marketing Manager
From: aloisi at uicc.com on Tue, Oct 3, 1995 11:13 AM
Subject: Ultra SCSI-2 ??? (You mean SCSI-3, SPI-2)
To: SCSI at symbios.com
Cc: aloisi at uicc.com

SPI-2 defines Low Voltage Differential.

Low Voltage Differential SCSI pin out is based on the single ended pin
out. The One exception is the diff sense line from Differential is used.

The Diff Sense line is ground for single ended devices or pulled high
with a 1K resistor and diode for the current Differential SCSI devices.

Low Voltage Differential checks the bus all devices monitoring the Diff
Sense line. The active LVDS terminators bias the line to a 1.3 Volts,
all devices on the bus monitor the Diff Sense Line to see if the voltage 
is between 0.7 and 1.9 Volts, if it is in this range it is Low Voltage
Differential. If it is below 0.6 Volts then the bus is single ended
and all devices revert back to single ended.

If an old style differential devices is on the bus the diff sense line
is pulled above 2.2 Volts and all devices turn off, because the pin
out is wrong, and there is a change for high voltage on the bus.

The current version of the Low Voltage Differential Document is 
X3T10-95/0269R5 Dated September 14, 1995. 

Thank you,
Paul Aloisi

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