X3T9.2/93-149 Rev 2
To:	John Lohmeyer		  
	Chairman, ANSI X3T9.2 (SCSI)

From:	Paul Aloisi,  Unitrode
	7 Continental Blvd
	Merrimack, NH 03054
	603 429-8687 Ph
	603 429-3460

Mail:	Aloisi@uicc.com

Date:	10, November 1993

Subj:	Proposed changes to SPI



	These proposed changes to SPI are derived from
discussions of document X3T9.2/93-149 in the SCSI working
group on November 10, 1993.  Specific working is proposed 
for the identified sections of SPI. 


1)  Change 6.2 3rd paragraph:

The TERMPWR and TERMPWRQ lines should be decoupled at 
each terminator with a low frequency capacitor 
(2.2 < C < 5.0 uF) and a high-frequency capacitor
(nominally 0,01 microfarad). 

	Rational:

If the capacitor size is not limited it causes major 
glitches on the Termpwr line during hot plugging.


2)  Change to Figure 9

Figure 9 Termination for differential devices.
Change Equal to or greater than 2.2 microfarads to 
(2.2 < C < 5.0 Microfarads) 



3)  Changes to Annex G section G.4 (relating to 
    device removal and insertion with current 
    I/O processes allowed)

	Add clauses (g) and (h) to read:

g) The power cycling in (d) includes the power cycle
   to the term power line on the device being removed 
   or inserted that naturally occurs.

   Switchable terminators in disconnect mode (i.e. not 
   being used as terminators) must not allow the high 
   impedance state at the device connector to be 
   disturbed during this intrinsic power cycle on the 
   term power line.   

h) Changes in TERMPWR voltage caused by charging the 
   capacitor on the TERMPWR line of the device being 
   removed or inserted shall not be transmitted to the 
   signal lines through the termination being used by 
   the active bus. 

	Implementor's note:  This specifically requires
        EITHER:

	 (1) that there be no capacitance greater than 
	     25 picofarads on the term power line of 
             the device -- passive terminators pass 
	     approximately 60% of the TERMPWR voltage 
             to the signal lines OR 

	 (2) that regulated terminators be used on the 
             active bus -- 	    
	     Limiting the capacitance on the TERMPWR 
             line of the device to 5.0 microfarads is 
             sufficient to meet this requirement if  
	     alternative-2 active terminators as 
             specified in SCSI-2 are used on the active 
             bus


4)  Changes to Section 7.1.1

	Add as f):

	f) The SCSI-2 220/330 terminator is not allowed 
           for SPI applications.

	Rational:

	This terminator does not adequately match the 
        impedance of the cables specified in SPI and 
        it passes 60% of TERMPWR noise to the data lines.


5)  Changes to Section 7.2.2

	Change b) to read:

	b) Shall conform to EIA RS-485 (ISO 8482-1982 
           TIA TR30.2) except that the common mode 
           voltage range shall be 0 to +5 volts instead
           of -7 to +12 volts.

	Rational:

	The common mode voltage requirements for EIA485
        are not possible without distroying SCSI cables.
        (The 7 ground wires in differential SCSI limits
        the common mode to transients or low voltages.)
        In addition these limits may cause excessive 
        power dissipation in the bus termination and 
        place unnecessary burden on the design and 
        testing of the differential transceivers.