May 9, 1991 X3T9.2/91-64 TO: X3T9.2 Committee FROM: David Steele NCR Corporation Microelectronics Products Division 1635 Aeroplaza Drive Colorado Springs, CO 80916 Telephone 719 596-5795 SUBJ: Proposal for Fast Single Ended Specification Introduction The current SCSI-2 specification for Fast SCSI is only defined for differential transceivers. There are many SCSI implementations which do not require the cable lengths supported by differential transceivers, but do require Fast SCSI performance. For some implementations, supporting the differential cable option is too expensive in terms of cost, board space, and power consumption. There is a limited class of applications for which Fast SCSI on a single ended (SE) cable is a viable option. A number of companies are now experimenting with Fast SE SCSI. However, taking Fast SE SCSI out of the laboratory and into the real world requires that some restrictions are placed on the cable and transceivers. In addition, with slight shifts in timing, the bus can be better optimized for the problems encountered in the SE environment. Brief Overview To briefly summarize what is being proposed, cable lengths and impedances are being restricted, SE transceivers are being more thoroughly specified, and synchronous data timings are being slightly shifted. For Fast SE SCSI, cable lengths will be limited to 3 meters. Cable impedances must be between 80 and 100 ohms. SE drivers must control slew rates and must provide active deassertion on REQ and ACK. SE receivers must have 300 mV of hysteresis and be tolerant of short transients on the bus due to signal reflections. By reducing the deskew budget associated with external differential transceivers, data timings have been shifted to provide more setup time for data settling due to bus reflections. Proposal Details Cable Specifications Cable length - 3 meters maximum. Simulations of 6 meter cables have demonstrated serious problems with Fast SCSI timings. The propagation delay of a 6 meter cable is about 30 ns, equal to the minimum pulse width of REQ and ACK. This results in signal transitions which do not completely settle before the next transition occurs. In some instances, simulations indicated that the bus would barely attain a 2 volt high level during deassertion. By limiting the bus to 3 meters in length, the propagation delay of the cable is 15 ns, or one half of the minimum pulse duration of REQ and ACK. Additional work is required to confirm this specification. Spacing between devices - 0.3 meters minimum. This specification is based on recommendations previously made to the X3T9.2 committee. It has been clearly demonstrated that spacing bus taps too closely results in a localized reduction in bus impedance. This reduction in impedance degrades the deassertion edge of REQ and ACK and can lead to the double clocking of data. Termination - Alternate 2 termination required. Alternate 2 termination has been demonstrated to provide improved deassertion transitions. Current experimentation being performed at NCR uses this type of termination. Signal wire placement - shall adhere to recommendations which have been previously made to the X3T9.2 committee. These recommendations reduce signal crosstalk by carefully choosing the placement of critical signals within the wire bundle. Cable impedance - 80 ohms minimum and 100 ohms maximum. This recommendation is based on work presented by Kurt Chan, HP; and Bill Spence, TI. It is also recommended that board traces on the attached SCSI devices be considered a part of the cable stub and adhere to this impedance range. The absolute impedance is not as critical as is the requirement that all cables in a system be within some range of one another. This reduces reflections and allows the line drivers to be better optimized in the protocol chip. Receiver Specifications Hysteresis - 300 mV minimum. This is an increase over the current 200 mV requirement. Receiver Specifications Continued Transient immunity - receiver must not respond to pulses of less than 5 ns duration on REQ or ACK. The filtering of transients on REQ and ACK has proven to be a very effective technique for dealing with cabling configurations that generate bus reflections. For the purposes of this specification, pulse widths are measured between 1.4 Volts crossing points. Line Driver Specifications Active deassertion - required on ACK and REQ. The only way that this technique can be recommended is if the second recommendation for controlling slew rate is also adopted. Active deassertion gets the first voltage step on the cable up out of the threshold region of the inputs. Care must be taken, however, not to introduce excessive ringing or current leakage. Active deassertion should be specified to source a minimum of 4 mA at 2.4 Volts. Slew rate control - SCSI signal slew rate is not to exceed 0.5 Volts/ns at any time during a transition. The controlling of SCSI driver slew rates has proven to be a very effective technique for dealing with cabling configurations that generate bus reflections and ring excessively. Timing Specification Fast Deskew - changed to 10 ns. Conservatively, 10 ns can be removed from the fast deskew budget when the SE transceivers are integrated into the protocol chip. By reducing the deskew requirement, this time can be budgeted elsewhere to better deal with the problems associated with fast SE. Fast SE Data Settle Delay - 15 ns. This is a new specification which has been set equal to one propagation delay on a 3 meter cable. The Fast SE protocol would require that data be set up one Fast Cable Skew Delay plus one Fast Deskew Delay plus one Fast SE Data Settle Delay before the assertion of REQ or ACK. This would allow the data more time to settle from bus reflections and would help prevent the requirement that active deassertion be applied to data and parity lines. This has a minimal impact on current designs, since they must already allow additional setup time to deskew external differential transceivers. Timing Specification Continued Fast Data Hold time - changed to 15 ns. 5 ns of the time gained by reducing the Fast Deskew Delay should be used to widen the data window. The current specification of 10 ns becomes more difficult to achieve when the requirement for transient filtering is imposed on REQ and ACK. This has a minimal impact on current designs, since they must already allow additional hold time to deskew external differential transceivers. Additional Comments Some additional issues are not completely addressed here due to insufficient information on which to base recommendations. Any input on the following issues, or any other issue, would be greatly appreciated. Device Input Capacitance: Can the SCSI-2 specified maximum of 25 pF per device be reduced for Fast SE? It has been suggested that this would be helpful if it were possible to do so. Board Trace Impedance: Problems have been encountered with SCSI devices exhibiting a low board trace impedance. Is it reasonable to require that these traces be considered to be a part of the stub and as such must meet the 80-100 ohm impedance requirement? Connectors: Are there connector possibilities which would reduce bus reflection problems, particularly between round and flat cable junctions? Pull-up Issues: Other than a minimum source current requirement, no restrictions were placed on the active deassertion implementation. How serious are the RFI and power considerations associated with driving SCSI signals to a 5 Volts deassertion level? Should additional restrictions be placed on the pull-up, such as a maximum VOH?