Datasheet

SCEA017
7–283 GTLP in BTL Applications
Background
FB+ devices are 5-V V
CC
BiCMOS translators that operate between TTL logic levels on the card
and BTL signal levels on the backplane. The backplane signals are generated by an open
collector and a termination network (see Figure 1). The saturation voltage of the pulldown
transistor and the forward voltage of the serially connected diode generate the output low-level
voltage of 1.1 V. The high level of 2.1 V comes from the termination resistor (R
TT
) connected to
the termination voltage of 2.1 V. R
TT
should equal the loaded-bus line impedance (Z), but the
effective dc resistance, as seen by the driver in a typical double-terminated backplane, is R
TT
in
parallel with R
TT
or one-half R
TT
. Matching R
TT
with Z ensures incident-wave switching and
improves signal integrity. FB+ devices can handle a current (I
OL
) up to 100 mA, which equals a
R
TT
of 20 Ω or an effective line impedance of 10 Ω (R = (V
OH
– V
OL
)/I
OL
). For safe detection of
logic levels, the inputs are designed with differential amplifiers and a fixed threshold at
1.55 V ± 75 mV, exactly in the middle of the voltage swing. FB+ logical-layer specification is,
according to IEEE Std 1194.1-1991 (BTL) and IEEE Std 896-1991 (FB+), describing node
management, live insertion, and profiles. However, the physical layer can be used stand-alone,
without the logical layer.
BUS
Receiver
R
TT
= Z
Line Driver
V
REF
= 1.55 V
V
CC
+2.1 V
Figure 1. Open-Collector Bus System Using BTL/FB+ Devices
The basic concept of the Gunning transceiver logic (GTL) bus (see Figure 2) is identical to the
BTL system, but in a CMOS technology and at a lower voltage level. Because of the missing
diode in the open-drain outputs, the low level is 0.4 V. With a high level of 1.2 V, the voltage
swing is reduced to only 0.8 V. The threshold is set in the middle of the voltage swing at 0.8 V by
the variable V
REF
input. With a drive capability of 40 mA, GTL devices can drive an effective
resistance of around 0.8 V/40 mA = 20 Ω, or a termination resistor R
TT
of 40 Ω in a
double-terminated backplane. As a result of the 0.4-V V
OL
level and the 40-mA I
OL
current drive,
the maximum output dc power dissipation of one output is 16 mW. These low-power drivers are
typically integrated into ASICs for point-to-point applications, and normally are not used in
heavily loaded multipoint applications.
Line Driver
R
TT
= Z
1.2 V Receiver
V
REF
= 0.8 V
Figure 2. Open-Drain Bus System Using GTL Devices