Termination and 3.3 Volt Systems X3T10/94-164R1

Paul D. Aloisi 603-429-8687 aloisi at uicc.com
Fri Jul 22 14:11:20 PDT 1994

3.3 Volt Systems SCSI Termination   X3T10/94-164R1
July 1994

Slide 1
3.3 Volt SCSI Termpwr
     3.3 Volts doesn't work with a diode drop.
        3.3 Volts minus tolerance minus a diode drop is less
        than 3.0 Volts.
        3.3 Volt Systems will connect to 5 Volt systems 
        requiring  a regulator for 3.0 to 5.25 Volt operation.
     Unidirectional Circuit Breakers are required for less 
     than 0.2 Volt drop.
        Section 7.3 Add a note Use a Unidirectional Circuit 
        Breaker instead of a diode and fuse for 3.3 Volt 
     Change SPI table 7 (Add a line) 3.3 Volt Single-ended
     3,00 VDC Min, 5,25 VDC Max, 1500 mA
Slide 2 
3.3 Volt Basics
     3.3 Volt systems require terminators that will work down
     to 3,0 VDC at the far end of the bus.
        Note: Standard regulated terminators only work from 
        4,0 to 5,25 VDC.
        Not all 3.3 Volt systems can provide current for the
        terminator at the far end. example 32 bit PCMCIA adapters
        have 1 Amp maximum available for the logic and termpwr.
        3.3 Volt systems must work with termpwr provided from 
        a 5 Volt System. 
        Add Special ICONS in ANNEX H to show 3.3 Volt and Limited
        termpwr current. (See Mailing for the proposed ICON.)
Slide 3
Termination with 3.3 Volt systems.
     SCSI-2 Alternative has an Option for 2.7 Volt regulated 
     termination. This will allow a 0,3 Volt drop out regulator
     to be used for the terminator with 110 ohm resistors meets
     the SCSI-3 24 mA at 0.2 Volts.
     Battery Systems will often require that termpwr is provided
     for the far end termination. This could be either 3,3 Volt or
     5,0 Volt systems. 
        All 3,3 Volt Terminators shall met all termination 
        requirements with Termpwr voltages from 3,0 to 5,25 VDC.
     Add a line to section 7.1.1 3,3 Volt Systems terminators must
     meet all specification with termpwr from 3,0 to 5,25 VDC.
Slide 4
Low Power Mode
     Battery systems with short buses do not need standard 
     termination current.
        Buses with less than 0,3 meters length within an
        enclosure can use reduce currents 1 mA typical. 
        Terminator must switch to normal termination currents
        when an external cable is attached.
        Signal line 50 on the 68 Pin connector or Singal line 22 
        on the 50 pin connector should be used to detect the 
        presents of an external cable.
     Add an Informative Annex to describe the 3.3 Volt systems and
     low/normal termination power modes. 

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