Proposed changes to SPI

Kevin Gingerich SN=Kevin_Gingerich%S=Gingerich%G=Kevin%TI at mcimail.com
Sat Jan 22 05:40:00 PST 1994


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Application message id:  TI.msg.252080
Grade of Delivery:  Normal

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  To  SCSI Reflector   X400
 
>From  Kevin Gingerich  GING
 
Subj  Proposed changes to SPI
 
Comments to Paul Aloisi's proposed change to SPI, item 4).
 
>From Paul's message:
4)  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 DC and
           -7 to +12 volts for transients less than 100 mS
           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.
 
Comments:
While it is true that you would test the fusing current of the cable grounds
with a steady-state potential between bus device grounds, 7.2.2 applies the
RS485 requirement to the signal outputs and not the cable. The electrical
characteristics of the RS485 drivers (and receivers) are not affected by band
limiting the common-mode voltage requirement as proposed. The arbitrary
transient duration of 100 ms might as well be dc as far as the silicon
connected to the bus is concerned.
 
/Kevin/




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