TO: X3T9.2 Committee X3T9.2-88-165R1 FROM: Kurt Chan, Hewlett-Packard 1/11/89 SUBJECT: Input Current, Resistor Tolerances Here are some modifications to 88-165 (updated for Rev 6A) that Bob Snively and I agreed upon to make my original suggestions more general. Square brackets are my comments - not to be included in Rev 7. 4.4.1.1 Output Characteristics All signals shall use open-collector or three-state drivers. Each signal driven by an SCSI device shall have the following output characteristics when measured at the SCSI device's connector: V(OL) = 0.0 volts dc to 0.5 volts dc at 48 milliamps sinking (signal assertion) V(OH) = 2.5 volts dc to 5.25 volts dc (signal negation) [this nomenclature added to be consistent with input nomenclature and for reference in 4.4.1.3 below. By the way, the 2.5V spec above is UNACHIEVABLE open-collector devices under worst-case conditions (see 88-165R0 for this analysis). Assuming 100 uA input leakage, 5% resistors, 4.25V termpwr, 28 AWG wire and 17 signals asserted, the actual V(OH) on the 18th signal achieved will be 2.2 volts dc, not 2.5. It appears, however, that transmission line effects have a much greater impact on signal integrity than this 0.2V of DC margin.] 4.4.1.2 Input Characteristics SCSI device signal receivers shall have the following input characteristics: V(IL) = 0.0 volts dc to 0.8 volts dc (signal true) V(IH) = 2.0 volts dc to 5.25 volts dc (signal false) Minimum input hysteresis = 0.2 volts dc 4.4.1.3 Additional Device Signal Characteristics [This text added because we wanted to make sure *output* leakages and capacitances were accounted for] SCSI device signals shall have the following electrical characteristics: When an SCSI device signal voltage is in the range of V(OL), the current flowing out of that signal on any SCSI device shall not exceed 0.4 milliamps. [what this says is that if a signal is being asserted, no device should be trying to actively negate the signal by more than 400 uA per device] When an SCSI device signal voltage is in the range of V(OH), the current flowing into that signal on any SCSI device shall not exceed 0.1 milliamps. [what this says is that if a signal is being negated, no device should be trying to actively assert the signal by more than 100 uA per device] The maximum capacitance introduced to a signal by an SCSI device shall not exceed 25 pF, measured at the device connector closest to the device stub. [Jeff Stai agreed with us that perhaps 20 pF was too stringent, considering: ~10 pF from receiver, ~5 pF from connector, leaving only 5 pF for the trace. Since we're wording this as a requirement and not a recommendation, we should make sure we're allowing enough headroom.] [Bob wanted the 25 pF spec to apply specifically to devices containing stubs - not to those devices which have an extension loop of SCSI cable with "real" devices hung off of that loop. These devices can have more than 25 pF, but they must notify users of how much cable they use internally for cable length budgeting purposes]. RESISTOR TOLERANCES ------------------- Add text to Rev 6A, section 4.4.1 to specify 5% resistors: (1) The termination of each signal shall consist of 220 ohms (plus or minus 5%) to the TERMPWR line and 330 ohms (plus or minus 5%) to ground at each end of the cable. UNPOWERED DEVICES ------------------ Add an implementors note recommending that the device electrical characteristics in 4.4.1.3 be met regardless of whether or not power is applied to the device. [John, I believe you drafted some words for this at Costa Mesa]