X3T9.2/87-195 REV. 2 January 8, 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 6, 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). In general, cables with this high of a characteristic impedance are hard to obtain; 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; how- ever, most available cables have a somewhat lower charac- teristic impedance.] See section 4.2.2.1 on differential cable for characteristic impedance requirements. To minim- ize discontinuities and cable reflections, cables of dif- ferent impedances should not be used in the same bus. Implementations may require trade-offs in shielding effec- tiveness, cable length, the number of loads, transfer rates, and cost to achieve satisfactory system 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 January 7, 1988 - 2 - 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 to 120 ohms +-10% SIGNAL ATTENUATION: .087 db max/meter at 5MHz. PAIR TO PAIR PROPAGATION DELAY DELTA: .20 nS/meter max. 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. IMPLEMENTORS NOTE: Fast data transfer times have been tested using the differential transceiver option with 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 max- imum of 200mV of Vcc difference. The maximum driver skew tested was 6nS (tPLH min. - tPHL max.) The max- imum receiver skew tested was 9nS (tPLH min. - tPHL max.). Fast data transfer timing parameters have not been tested for the single ended transceiver option. 4.8.1 Fast Assertion Period (30 nanoseconds) When using the Fast SCSI option, 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) When using the Fast SCSI option, the maximum difference in propagation time allowed between any two SCSI bus January 7, 1988 - 3 - signals when measured between any two SCSI devices while using synchronous data transfers between a 100nS and 200nS transfer period. 4.8.3 Fast Deskew Delay (20 nanoseconds) When using the Fast SCSI option, the minimum time required for deskew of certain signals while using syn- chronous data transfers between a 100nS and 200nS transfer period. 4.8.4 Fast Hold Time (10 nanoseconds) When using the Fast SCSI option, the minimum time added between the assertion 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) When using the Fast SCSI option, 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..... January 7, 1988 L57%