National Semiconductor Corporation X3T9.2/87-195 REV. 3 January 12, 1988 TO: ANSI X3T9.2 Committee FROM: Jim Schuessler SUBJECT: Proposed Text for Fast SCSI Changed as per recommendations of San Jose Working Group, January 8, 1988. This paper consolidates documents X3T9.2/87-138, -148, -175 by restating and editing proposed text in those documents into something worthy of direct inclusion in the SCSI-2 draft. The proposal consists of three main alterations: New cable specifications similar to IPI, Fast SCSI timing parameters, and Selection of those timing parameters with descriptions in the Synchronous Transfer section. Please consider the following sections for inclusion in the SCSI-2 draft. 4.2 Cable Requirements An ideal impedance match with cable terminators implies a cable characteristic impedance of 132 ohms (single-ended option) or 122 ohms (differential option). [delete: In gen- eral, cables with this high of a characteristic impedance are not available; however,] Impedances that are somewhat lower are satisfactory. A characteristic impedance of 100 ohms plus or minus 10% is recommended for unshielded flat or twisted pair ribbon cable. [delete: A characteristic impedance greater than 90 ohms is preferred for shielded cables; however, most available cables have a somewhat lower characteristic impedance.] See section 4.2.2.1 on differen- tial cable for characteristic impedance requirements. IMPLEMENTORS NOTE: To minimize discontinuities and cable reflections, cables of different impedances should not be used in the same bus. Implementations may require trade- offs in shielding effectiveness, cable length, the number of loads, transfer rates, and cost to achieve satisfactory sys- tem operation. 4.2.2 Differential Cable Add after last paragraph: 4.2.2.1 26AWG and 28AWG Twisted Pair Cable Cables A and B shall consist of conductors of 26AWG or 28AWG. The cable may have an overall shield, suitable for termination in a metal shielded connector. Such overall shield serves the purpose of a signal shield. The two wires of a pair shall be connected to the same signal, one to the positive and the other to the nega- tive signal. The cable shall have the following electrical charac- teristics: CHARACTERISTIC IMPEDANCE: 100 +-10% to 120 ohms +-10% SIGNAL ATTENUATION: 0.095 db max/meter at 5MHz. PAIR TO PAIR PROPAGATION DELAY DELTA: 0.20 nS/meter max. DC RESISTANCE: 0.230 ohms max./meter at 20 degrees C. Add after 4.7.15: 4.8 Fast Synchronous Transfer Option: Fast synchro- nous transfers are used when devices negotiate a syn- chronous transfer period between 100nS and 200nS. Dev- ices which negotiate a synchronous transfer period greater than 200nS use timing parameters specified in section 4.7. When a fast synchronous transfer period is negotiated, those specific times redefined in this section are used, those not redefined remain the same. The minimum synchronous transfer period is 100nS. 4.8.1 Fast Assertion Period (30 nanoseconds) This value is the minimum time that a target shall assert REQ while using synchronous data transfers between a 100nS and 200nS transfer period. Also, the minimum time that an initiator shall assert ACK while using synchronous data transfers between a 100nS and 200nS transfer period. 4.8.2 Fast Cable Skew Delay (5 nanoseconds) This value is the maximum difference in propagation time allowed between any two SCSI bus signals when measured between any two SCSI devices while using syn- chronous data transfers between a 100nS and 200nS transfer period. 4.8.3 Fast Deskew Delay (20 nanoseconds) This value is the minimum time required for deskew of certain signals while using synchronous data transfers between a 100nS and 200nS transfer period. 4.8.4 Fast Hold Time (10 nanoseconds) This value is the minimum time added between the asser- tion of REQ or ACK and the changing of the data lines to provide hold time in the initiator or target, respectively, while using synchronous data transfers between a 100nS and 200nS transfer period. 4.8.5 Fast Negation Period (30 nanoseconds) This value is the minimum time that a target shall negate REQ while using synchronous data transfers between a 100nS and 200nS transfer period. Also, the minimum time that an initiator shall negate ACK while using synchronous data transfers. between a 100nS and 200nS transfer period. Add before first paragraph in sec. 5.1.5.2 Synchronous Transfer: 5.1.5.2 Synchronous Data Transfer (Optional) Synchronous data transfer is optional, and uses two sets of timing parameters which depend on the nego- tiated transfer period. See sections 4.8 and 5.5.5. Continue paragraph by replacing first line in existing first paragraph with: Synchronous transfers may only be used in the data phase if previously agreed to by the initiator and tar- get through the message system..... ____________________________________________________________ Appendix ? Implementing Transfer Rates above 5MHz.: Synchronous transfer rates using a transfer period between 100nS and 200nS are known as the "Fast SCSI" option. Fast data transfer times have been tested using the following skew budget with the differential transceiver option using 25m of 28AWG. twisted pair cable as specified in 4.2.2.1. The transceivers were subjected to a maximum temperature difference of 25 degrees celsius and a maximum of 200mV of Vcc differ- ence. ________________________________________________ | FAST SCSI | |______________________________________________| | # | parameter | +-budget| |___|_______________________________|__________| | a | clock offset | 5 | | b | transmitting logic skew | 3 | | c | foil delay | 1 | | d | transmitter prop. delay skew | 6 | | e | foil delay | 1 | | f | drop cable prop. delay | 1 | | | | | | | CONNECTOR | | | | | | | g | external cable - skew | 5 | | | between pairs | | | h | distortion due to cable | 1 | | | imbalance | | | i | distortion due to | 2 | | | intersymbol interference | | | j | bias distortion | 2 | | | | | | | CONNECTOR | | | | | | | k | drop cable prop. delay | 1 | | l | foil delay | 1 | | m | receiver skew | 9 | | n | foil delay | 1 | | o | logic setup/hold | 5 | |___|_______________________________|__________| | TOTAL 44nS | |______________________________________________| Mapping the above jitter or skew budget to the SCSI format in section 4.7 and 4.8 is done in the following table. _____________________________________________ | Table # | parameter in 4.7-8 | value | |_________|________________________|________| | g | Fast Cable Skew Delay | 5 | | h - n | Fast Deskew Delay | ~20 | | o | Fast Hold Time | ~10 | | * | Fast Assertion Period | 30 | | * | Fast Negation Period | 30 | |_________|________________________|________| ~ Values preceded with "~" are rounded up from the numbers shown in the previous table. * The assertion and negation pulse widths are derived from isolated pulse measurements and represent a comfortable minimum pulse width. The maximum driver skew allowed was 6nS (tPLH min. - tPHL max.) and the maximum receiver skew tested was 9nS (tPLH min. - tPHL max.). Values greater than these could be used if other numbers could be reduced: the sum is what is important. Fast data transfer timing parameters have not been tested for the single ended transceiver option. ion of REQ or ACK and the changing of the data lines