X3T9.2/92-122 MEMORANDUM -- 18 May 1992 TO: John Lohmeyer, Chairman, X3T9.2 FROM: Bill Spence, Chair, SPI WG SUBJECT: Proposal for Additional Annex for SPI Standard Among the plans for the SPI document, two placed a special requirement on the Annexes (Appendixes): (1) that much of the material previously presented in Implementors Notes be removed from the body to an Annex, and (2) that additional help be provided to support a successful implementation. The material below is offered as a first step toward those goals. TERMINATOR, IMPEDANCE, CROSSTALK, AND BUS LENGTH CONSIDERATIONS SCSI terminators serve two basic functions: to establish the voltage of passively negated lines, and to establish the currents in asserted lines. In some cases, they can also be used to match line impedances to eliminate reflections. Single-Ended Alternative For SCSI signal transfers to achieve high reliability, SCSI signals at the using receivers must show monotonic transitions from above 2.0 to below 0.8 volts, or vice-versa, with transition times in the 5 - 10 ns range. For SCSI bus signals to maintain these standards when encountering lumped capacitances and stubs, it is helpful for the characteristic impedance Zo of the bus conductors to be relatively low. But for signals to be able to achieve acceptable initial levels when released in passive negation, it is helpful for Zo to be relatively high. The higher the assertion current furnished by the terminators, the lower the acceptable level of Zo. With the full 24 ma allowed, the minimum allowed Zo of 73 ohms is generally satisfactory. But note that the allowed 73 ohms applies to the lowest impedance conductor used, not to the average for all the conductors in the cable. The first widely used terminator comprised 220/330 ohm resistor pairs, SCSI-1 Alternative 1. This terminator will meet the 2.5 volt minimum voltage requirement with any TERMPWR voltage of 2.0 volts or more but has two important shortcomings: 1. Because it has no regulation, its effect on bus signals varies with any variations in TERMPWR voltage. 2. Even with a nominal 4.7 volts of TERMPWR voltage, it provides only about 20 mA into an asserted line held at 0.2 volts. Because of these shortcomings, the use of this terminator is deprecated. The Alternative 2 terminator of SCSI-2--a regulated 2.85 volt circuit node linked to each line through 110 ohm resistors--does not suffer such shortcomings. Additional margin in the passively negated signal is initially achieved if one of the two devices in a data exchange is at or is very near an end of the bus. In this case, the terminator at this end can be directly effective in improving the initial passive negation levels. Its effectiveness is enhanced if the current it sources at all signal levels is increased over what can be achieved with a simple linear circuit, such as either of those presented above--i.e., if the terminator displays a somewhat current-sourcing characteristic. Greater margin yet in the passively negated signal is achieved by terminators which deliberately exceed the allowed 24 mA limit on current sourced into an asserted line. Although there must be a reduction in driver MTBF with this practice, experience indicates that it generally is very slight. Such extra current provides protection against lower than optimal cable impedances and possible other deficiencies in implementation. It can, however, hurt the assertion edges of signals received at the ends of long buses. Crosstalk noise in the bus is not a problem with flat ribbon cable and is best best controlled by conductor placement (clocks in the center, data around the periphery) in round, twisted-pair cables. Any other correctives may be harmful by increasing signal skew. The 3 m and 6 m length limits suggested for 10 and 5 megatransfer per second transfers, respectively, are aimed at promoting successful implementations. Where circumstances present a need for longer bus operation, special emphasis must be paid to the guidelines suggested above and in the standard: - Limit lumped capacitance - Limit stub length and crowding - Control cable impedance - Control current sourced into asserted lines - Control crosstalk effects - Provide input glitch rejection in receivers - Place initiators at or very near bus ends, and/or - Use active-negation drivers on clock and data lines Differential Alternative The extra cost, power, and space requirements of differential implementations are compensated for by superior reliability margin. This arises from the noise rejection properties of differential signals, the approximately 1 volt noise margins provided, and the fact that all differential drivers to date have provided active negation. Because of this, there has developed no demand for improvements in the original differential terminator, as presented in Figure 7. The requirement for good high-frequency bypass right at the terminator is the same as in single-ended, however. Cross-talk effects can be significant in long flat ribbon cables, and for this reason the use of twisted pairs in ribbon cables is encouraged. In round twisted-pair cables, crosstalk has generally not been a significant problem. The ratio of impedances in a cable measured in single-ended and differential modes corresponds well to the ratio of impedances set forth in this standard for the two modes of operation. Because of this, and because of the relative immunity to cross-talk, round twisted-pair cables optimized for single-ended service are suitable for differential service also. There is generally no need to create two separate sets of cables.