National X3T9.2/87-195 Semiconductor November 9, 1987 Corporation TO: ANSI X3T9.2 Committee FROM: Jim Schuessler SUBJECT: Proposed Text for Fast SCSI 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 ar 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 sections on differential cable for characteristic impedance requirements. To minimize discon- tinuities and cable reflections, cables of different impedances should not be used in the same bus. Implementa- tions may require trade-offs in shielding effectiveness, 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 28AWG Twisted Pair Cable Cables A and B shall consist of conductors of 28AWG. The cable may have an overall shield, suitable for ter- mination in a metal shielded connector. Such overall November 9, 1987 - 2 - 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 negative sig- nal. 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. 4.2.2.2 26AWG Twisted Pair Cable Cables A and B shall consist of conductors of 26AWG. The cable may have an overall shield, suitable for ter- mination 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 negative sig- nal. 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. 4.7.17. Fast Assertion Period (45 nanoseconds) When using the Fast SCSI option, the minimum time that a target shall assert REQ while using synchronous data transfers at 100nS transfer period. Also, the minimum time that an initiator shall assert ACK while using synchronous data transfers. 4.7.18. Fast Cable Skew Delay (5 nanoseconds) When using the Fast SCSI option, the maximum difference in propagation time allowed between any two SCSI bus signals when measured between any two SCSI devices at 100nS transfer period. 4.7.19. Fast Deskew Delay (20 nanoseconds) November 9, 1987 - 3 - When using the Fast SCSI option, the minimum time required for deskew of certain signals at 100nS transfer period. The main components of this deskew delay are cable distortion and receiver skew. 4.7.20. 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 at 100nS transfer period. 4.7.21. Fast Negation Period (45 nanoseconds) When using the Fast SCSI option, the minimum time that a target shall negate REQ while using synchronous data transfers at 100nS transfer period. Also, the minimum time that an initiator shall negate ACK while using synchronous data transfers. To be placed after 4.7.21. IMPLEMENTORS NOTE: Fast data transfer times have been tested using the differential transceiver option with 25m of 28GA. twisted pair cable as specified in 4.2.2.1. The tran- sceivers wee subjected to a maximum temperature difference of 25 degrees celsius and a maximum of 200mV of Vcc differ- ence. The maximum driver skew tested was 6nS (tPLH min. - tPHL max.) The maximum receiver skew tested was 9nS (tPLH min. - tPHL max.). Fast SCSI Add before first paragraph in sec. 5.1.6.2 Synchronous Transfer: 5.1.6.2 Synchronous Data Transfer (Optional) Synchronous data transfer is optional, and uses several timing parameters specified in section 4.7 which have "Fast" counterparts. Up to a 200nS transfer period, the normal Cable Skew Delay, Deskew Delay, Hold Time, Assertion Period and Negation Period are used. When the transfer rate exceeds 200nS, a linear curve shall be followed between the normal timing parameter and it's Fast counterpart at 100nS. The minimum Fast SCSI transfer period is 100nS. For example if a 150nS transfer period is desired, and the minimum Deskew Delay is needed, the result would be 32.5nS. November 9, 1987 - 4 - Replace first line in existing first paragraphwith: Synchronous transfers may only be used in the data phase if previously agreed to by the initiator and tar- get through the message system..... Jim Schuessler. END - OF - FILE