DN34 - Active Termination for SCSI-2 Bus

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
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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
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