14 May 1992 To: Bill Spence, Chair, SPI WG CC: John B. Lohmeyer, X3T9.2 Chairman Sb: Proposed change to SPI Ref: draft working document SCSI-3 Physical Interface Bill, upon reviewing the subject document I would like to propose the following change to the specification. I can find no basis for the single- ended specifications in the differential portion of the physical interface. It is possible to meet the requirements of EIA RS-485 1983 and the minimum VOD of 1 V without meeting these single-ended requirements. This change provides sufficient specification for data interchange and interoperability on a differential SCSI bus without undue restrictions on the interface integrated circuits. Regards, Kevin Gingerich, Texas Instruments, Inc. P.O. Box 655303 MS 8206 Dallas, TX 75265 Tel: (214)997-3378/Fax: (214)997-3165/Internet: 4307725@mcimail.com Change from: 6.2.1 Differential Output Characteristics Each signal sourced by an SCSI device shall have the following output characteristics when measured at the SCSI device's connector: VOL (Low-level output voltage) = 1.7 V maximum at IOL (Low-level output current) = 55 mA VOH (High-level output voltage = 2.7 V minimum at IOH (High-level output current = -55 mA VOD (Differential output voltage = 1.0 V minimum with common-mode voltage ranges from -7 to +12 volts dc. VOL and VOH shall be measured between the output terminal and logic ground of the SCSI device. The output characteristics shall additionally conform to EIA RS-485 1983. to: 6.2.1 Differential Output Characteristics Each differential signal sourced by a SCSI device shall comply with EIA RS-485 1983 and provide a minimum differential output voltage, VOD, of 1.0 V when the signal is asserted or negated with the following test circuit. |\ | \ | +SIGNAL 165 ohms +/-5% | \ |------------|---------/\/\/\/------| | \ | + / | | \| VOD \ 75 ohms +/-5% |------| | /| - / | | | / |O-----------|---------/\/\/\/------| + | / | -SIGNAL 165 ohms +/-5% -7