Terminators
WHAT ISA TERMINATOR?
A terminator is a device which
terminates the end of a signal line.
Terminators can be internal to the
system, or they can be in a
separate configuration known as a
“TERMINATOR”. This is the preferred
configuration for expandable bus
systems due to its versatility.
Physically, terminators are connectors
with a resistor network or a resistor
and current-voltage regulation
network encased in a protective
overmold or shell assembly.
WHY USE A TERMINATOR?
SCSI terminators have two basic
functions; 1) establish the proper
voltage and current for signal line
operation; 2) match the line
impedance and reduce reflection.
1. A terminator is an important
working partner for a driver. When a
driver pulls down the signal, a voltage
low (true state) terminator supplies
proper current to maintain the voltage
level and when a driver pulls up the
signal voltage high (negate), a
terminator helps a driver make a
quick and smooth negation.
2. Termination is needed at the two
ends of the SCSI bus. The
impedance mismatches are generally
of greatest magnitude at the ends of
the bus. A terminator matches a bus
characteristic impedance and
minimizes the reflection.
GENERAL TYPES OF TERMINATION
There are two general methods of
accomplishing termination: passive
and active.
Passive termination systems make
use of fixed value resistors that place
a matching resistance at the end of
each signal line. This method is
satisfactory for many applications.
However, as signal speeds increase
165
Thomas & Betts Electronics/OEM Division Tel: 901-682-7766 Fax: 901-680-5200
INTRODUCTION TO
TERMINATORS
and noise margins decrease, passive
termination may no longer assure a
reliable signal transmission.
Active termination is the solution to
this challenge. It reacts to an active
negation driver and helps to make a
clean monotonic wave shape in a
high speed signal transfer by abilities
of sourcing and sinking current and
protecting a system by limiting
maximum current.
GENERAL
CONFIGURATION OF
TERMINATORS
Terminators are available in two
general configurations: feedthrough
and end-of-line. A feedthrough
termination typically installs between
an equipment I/O port and a ribbon
cable bus connector. This permits a
quick and simplified change in
peripheral devices within an existing
system configuration. This type of
configuration is used if there is no
provision for a pluggable terminator.
Although these terminators generally
appear as devices with an input and
output port, they may also have “no
ports”, in which case they are
crimped directly to the cable.
End-of-line terminators are installed
at the end of a line, and do not
continue the bus. The devise using
this type of terminator must have
both an input and output port.
In addition, terminators may be of a
shielded or unshielded construction.
Shielded terminators are generally
required when a bus in used
“between” equipment cabinets. An
unshielded terminator, conversely, is
intended for use “within” an
equipment cabinet.
IDC INLINE-UNSHIELDED
TERMINATORS
(SL SERIES)
Terminators
169
Thomas & Betts Electronics/OEM Division Tel: 901-682-7766 Fax: 901-680-5200 Dimensions shown are for reference only. Dimensions are in millimeters (inches).
LR92984 E60980
P
HYSICAL
P
ROPERTIES
I
NSULATION
M
ATERIAL
:Thermoplastic
rated UL 94V-0.
C
ONTACTS
:Copper alloy; Tin-lead over nickel
plating overall.
E
LECTRICAL
P
ROPERTIES
W
ATTAGE
R
ATING
:1/8 Watt
M
AXIMUM
O
PERATING
V
OLTAGE
:50 volts
E
NVIRONMENTAL
P
ROPERTIES
T
EMPERATURE
R
ATING
:0˚C to 70°C
C
ONFIGURATION
:
Internal, end of line, unshielded
A
PPLICATION
:
SCSI-1 and SCSI-2 systems. Should be used in
application within an enclosure.
I
NSTALLATION
T
OOL FOR
S
LIMLINE
IDC T
ERMINATOR
:
P/N 779-3196
R
ECOMMENDED
C
ABLES
171-50
173-50
200-50A
201-50
Single-Ended
Differential
Forced Perfect
PART NUMBER STYLE
TRM050SL Single-Ended
TRM050SLAN Active Negation
TRM050SLFPT Forced Perfect
TRM050SLDIF Differential
Active Negation
