Active Termination for SCSI-2 Bus – Design Note 34 Sean Gold Overview of SCSI-2 The SCSI-2 bus1 is an interface for computers and instrumentation that communicate over small distances — often within the same cabinet. Like GPIB (IEEE 488), SCSI’s hardware and software specifications are designed to coordinate independent resources such as disk and tape drives, file servers, printers, and other computers. SCSI-2 is a bidirectional bus, which must be terminated at both ends to 2.85V (Figure 1). The terminators are needed because SCSI-2 uses simple open collector output drivers in its transceivers. Terminators link communicating devices to the supplies, and roughly match the transmission line’s characteristic impedance. When the load to the bus increases, the role of the termination network becomes more important for maintaining signal integrity at high data rates. An active termination design is now a part of the SCSI-2 standard and is presented here in-depth. the bus at regular distances along the cable. Any two devices can terminate the cable, but bit error rates are minimized with the terminators attached only at the ends. Local capacitive loading is low under these conditions, making the transmission line more consistent with fewer discontinuities. SCSI-2’s key specifications are repeated from the ANSI standard in Table 1. Table 1. Single Ended SCSI-2 PARAMETER VALUE Termination Supply 4.25 < TERMPWR < 5.25 COMMENTS 0.9A Typical 1.5A Worst Case Logic Supply VOUT = 2.85V @0.5A Per Terminator 2.6 < VOUT < 2.9 Data Rate 5M Transfers/Sec. Six Meters Max. Cable Impedance 110Ω 80 < ZO < 140 Nominal The single ended SCSI-2 bus is limited to six meters in length, and supports variable speed communication up to 5M transfers/sec. The bus nominally uses 18 data lines which defines the loading requirements for the terminators, because each output driver can sink at most 48mA. Up to eight SCSI devices can access Transceivers TTL Compatible Negative True Logic 5V = 0, 0V = 1 Signal Levels 0 < VOL < 0.5 2.5 < VOH < 5.25 VIL < 0.8 2.0 < VIH 0.2 < Hysteresis Note 1: SCSI-2 = Small Computer System Interface Version 2, pronounced “Scuzy-2.” The complete specifications standard is available through ANSI #X3T9.2. Short Circuit Current 48mA/Transceiver –0.4mA < IIL < 0mA 0.0mA < IIH < 0.1mA Based on Old TTL Spec L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TERMPWR VT SOURCE SUPPLY TERMINATOR VT SOURCE SUPPLY x18 DATA LINES SCSI DEVICE TERMINATOR SCSI DEVICE UP TO 8 Figure 1. Global View of the SCSI-2 Bus 05/90/34_conv DN034 F01 Shortcomings of Passive Terminators The resistive voltage divider shown in Figure 2 is commonly used to terminate the SCSI bus. Multiple power sources are allowed to connect to the SCSI cable. Each source is protected with a Schottky diode to prevent damage from reverse currents. The resulting termination power signal, TERMPWR, is not well regulated — subject to variations in source supplies and protection diodes, as well as ohmic losses. Unfortunately, these changes in TERMPWR translate directly to the bus through the resistive divider, which degrades noise margins. 1N5817 TERMPWR 10μF R1 220 R2 330 10μF 220 SIGNAL WIRE 110Ω 330 DN034 F02 DEVICE 1 DEVICE 2 Figure 2. Passive Termination The low values for R1 and R2 reflect a compromise between driver sink current and impedance matching the signal lines. Normally, high resistances would be desirable to minimize driver sink current. Yet, the terminator should match the signal line’s 110Ω characteristic impedance, and the bus’s quiescent state must be above the TTL logic threshold. It is not possible to meet all of these objectives simultaneously. The SCSI standard suggests R1 = 220Ω and R3 = 330Ω. The resulting bus voltage is 3V with 132Ω impedance, which is mismatched to the nominal 110Ω cable impedance. The Schottky diode aggravates the mismatch because it presents a poor AC ground. In addition to these problems, the small resistors draw 300mA Q-current from TERMPWR, assuming 18 signal lines with the bus inactive. Active Terminators The active terminator shown in Figure 3 uses an LT®11172.85 low dropout regulator to control the logic supply. The LT1086’s line regulation makes the output immune to variations in TERMPWR. After accounting for resistor tolerances and variations in the LT1117’s reference, the absolute variation in the 2.85V output is only 4 percent over temperature. When the regulator drops out at TERMPWR – 2.85V = 1.25V, the output linearly tracks the input with a slope of 1V/V. Signal quality is quite good because the 110Ω series resistor closely matches the transmission line’s characteristic impedance, and the regulator provides a good AC ground. In contrast to the passive circuit, two LT1117s require only 20mA quiescent current. For the power levels in this application, the LT1117 does not need a heat sink, and is available in low cost, space saving SOT-223 surface mount packages. Beyond solving basis signal conditioning problems, the LT1117 handles fault conditions with short circuit current limiting, thermal shutdown, and on-ship ESD protection circuitry. TERMPWR 1N5817 10μF TANTALUM LT1117-2.85 IN OUT GND 110Ω, 2% 22μF TANTALUM 0.1μF CERAMIC 110Ω 110Ω, 2% 110Ω 18-27 LINES 110Ω, 2% 110Ω DN034 F03 Figure 3. Active Termination Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn34f_conv IM/GP 0590 165K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1990