X3T9.2/88-060



                                                                  May 13, 1988

To:       X3T9.2 Membership


From:     John B. Lohmeyer, X3T9.2 Chairman

Subject:  Active Termination for Differential B Cables


I accepted an action item at the May working group meeting to document the 
need for active termination on differential B cables.  The current document 
only requires the REQB and ACKB signals in Cable B to be terminated with the 
powered termination circuit.  All of the DATA BUS signals in cable B are 
terminated with a passive shunt resistor.

The present requirements are fine for SCSI-2, but quite possibly will have to 
change in SCSI-3 if we accept the AT&T proposal to permit arbitration with 
additional device IDs on Cable B.  I would rather not go through the migration 
problems then when we can avoid the problem altogether now by specifying a 
powered termination.

This proposal does not affect single-ended systems which already have powered 
termination on Cable B.

The power requirements on TERMPWRB for differential systems would increase 
from today's approximate 0.1 Amps to 0.9 Amps for 16-bit systems and 1.3 Amps 
for 32-bit systems.  This is a modest increase for the capability to double or 
quadruple the number of devices on the SCSI bus.  

To implement this proposal, the following changes would be required in Section 
4 of SCSI-2:



4.4.2. Differential Alternative

  All signals consist of two lines denoted +SIGNAL and -SIGNAL.  A signal is 
true when +SIGNAL is more positive than -SIGNAL, and a signal is false when -
SIGNAL is more positive than +SIGNAL.  All assigned signals of Cable A and 
REQB and ACKB of Cable B described in section 4.6 shall be terminated at each 
end of the cable with a powered terminator network as shown in Figure 4-6.  
The remaining signals of Cable B shall be terminated at each end of the cable 
with a resistive terminator as shown in Figure 4-7.


4.4.3. Terminator Power

  Differential SCSI devices providing terminator power on cable B shall have 
the following characteristics:

  VTerm = 4.0 volts dc to 5.25 volts dc
          1300 milliamps minimum source drive capability
          1.0 milliamp maximum sink capability 




                 Figure 4-6: Powered Termination for Differential Devices





                 Figure 4-7: Passive Termination for Differential Devices