December 7, 1991                           X3T9.2/91-64 REV 2

  TO: X3T9.2 Committee

FROM: David Steele
      NCR, Microelectronics Products Division
      1635 Aeroplaza Drive
      Colorado Springs, CO 80916
      Telephone: (719) 596-5795 ext. 363

SUBJ: Fast Single-Ended and Other Recommended Changes to
      SCSI-3 Parallel Interface




5.1 Physical Description
Add to end of second paragraph or add to Annex A - When
mixing SCSI devices of different widths, care must be taken
not to exceed the skew allowances provided by the Cable Skew
Delay and Deskew Delay SCSI bus timing values (see 7.0).  To
accomplish this may require restricting SCSI device input
capacitance, stub length, cable length and the number of SCSI
devices to less than the specified maximum allowable value
and close adherence to the cable recommendations made in
section 5.2.

Modify last sentence of fourth paragraph to read - Use of
single-ended drivers and receivers with the fast synchronous
data transfer option is recommended only for a restricted set
of conditions.  Recommendations for single-ended systems
which use the fast synchronous data transfer option are made
in sections 5.2.3, 6.1.1 and 6.1.2.


5.2 Cable Requirements
Modify first paragraph to read - The characteristic impedance
of the cable should be no less than 80 ohms and no more than
100 ohms.  There are successful single-ended implementations
using cables with characteristic impedances outside of this
range.  However, system integrity in single-ended
implementations is improved when the characteristic impedance
of the cable is between 80 and 100 ohms.  Cable parameters
other than cable impedance are critical to system integrity. 
See Annex (TBD) (use document X3T9.2/90-134 as basis for
appendix) for information concerning the measurement of cable
parameters.









5.2.3 Cable Requirements for Fast Synchronous Data Transfer
Modify characteristic impedance to read -
Characteristic Impedance: 80 to 100 ohms

Add to end of section 5.2.3 - In single-ended systems which
use the fast synchronous data transfer option, it is
particularly important to follow the cable recommendations
made in section 5.2.  In addition, it is recommended that
cable lengths be restricted to less than 3 meters.  There are
successful single-ended implementations using cables longer
than 3 meters.  However, to maintain system integrity may
require restricting SCSI device input capacitance, stub
length and the number of SCSI devices to less than the
specified maximum allowable value and close adherence to the
cable recommendations made in section 5.2


6.1.1 Single-Ended Output Characteristics
Add to end of section 6.1.1 - It is recommended that
single-ended systems which use the fast synchronous data
transfer option have the following output characteristics
when measured at the SCSI device's connector:

all signals;
Slew Rate <= 0.5 volts/ns

See Annex (TBD) for information concerning the measurement of
slew rate.  (The intention here is to provide a simple test
circuit for measuring slew rate.)

ACK, REQ and DATA BUS signals;
It is recommended that the ACK, REQ and DATA BUS signals use
three state drivers to actively negate these signals.  The
three state output drivers for these signals should have the
following electrical characteristics when driving in the
negated state:

IOH(High-level output current) = 7.12 mA maximum at 
                                 VOH = 3.24 volts dc

IMPLEMENTORS NOTE: 
The recommended IOH maximum is based on maintaining
compatibility with the most common implementations of
Alternative 2 Termination (see 6.1) on a P Cable with the
ACK, REQ and DATA BUS signals employing three state drivers. 
Implementations for use on an A Cable only can increase IOH
to 8.54 mA maximum at VOH=3.32 volts dc.  These values for
IOH assume that at least 3 signals are in the asserted state
on a P Cable (BSY and two Data or Parity bits) or 2 signals
are in the asserted state on an A Cable (BSY and one Data or
Parity bit).  For this assumption to hold true, devices
employing three state drivers should not actively negate the
DATA BUS unless parity is valid.



6.1.2 Single-Ended Input Characteristics
In order to support more than 8 SCSI devices on a cable, the
IIL and IIH specification should be changed as follows:

IIL(Low-level input current)=-20 to 0 uA at VI=0.5 volts dc
IIH(High-level input current)=0 to 20 uA at VI=2.7 volts dc

Add to end of section 6.1.2 - It is recommended that
single-ended systems which use the fast synchronous data
transfer option have the following input characteristics:

all signals;
Minimum input hysteresis = 0.3 volts dc

ACK and REQ signals;
It is recommended that SCSI receivers on the ACK and REQ
signals be capable of tolerating the narrow voltage
transients that can occur during the negation transition of
these signals.  These transients are frequently the result of
voltage reflections from stub capacitances and the mixing of
cables of different characteristic impedances.  As a minimum,
it is recommended that the ACK and REQ input receivers not
change logic state as the result of a voltage swing on the
input signal of amplitude less than or equal to 1.0 volt and
is of less than or equal to 10 ns in duration.


6.4 SCSI Bus
Add after second paragraph - When more than 8 SCSI devices
are used on a cable, signal quality may be degraded.  This is
particularly a concern in single-ended implementations. 
There are successful single-ended implementations using more
than 8 SCSI devices on a cable.  However, to maintain system
integrity may require restricting SCSI device input
capacitance, stub length and cable length to less than the
specified maximum allowable value and close adherence to the
cable recommendations made in section 5.2


6.5.1 Signal Values
Add to end of paragraph - See section 6.1.1 for additional
recommendations concerning the use of active negation
drivers.