Datasheet
7–193
Introduction
This application report examines the requirements for a low-swing interface in high-speed digital systems and how well this need
is addressed by two interface standards: backplane Transceiver Logic (BTL) and Gunning Transceiver Logic (GTL). Both
interface standards attempt to improve the performance of high-speed digital systems by reducing the difference between the
logic high-voltage level and the logic low-voltage level.
A comparison of various performance criteria, such as power consumption, noise immunity, capacitive loading, speed, and
packaging, shows that GTL and BTL provide a compelling solution in both point-to-point and backplane environments.
Guidelines for system designs using Texas Instruments (TI) GTL and BTL products are addressed, including associated voltage
supplies and proper termination techniques.
Test Setup
The TI GTL16612 and FB1650 were used to study the various performance levels. A backplane-like design has been established
to perform the laboratory work supporting this application report. Four boards with 2-in. stubs and 50-Ω interconnecting
transmission lines were used to simulate the backplane environment. A 50-MHz frequency was used unless otherwise noted. The
output supply voltage (V
TT
) was supplied through a resistor at each end of the backplane (50-Ω to 1.2 V for GTL and 33-Ω to
2.1 V for BTL) for both families as specified in both IEEE (BTL) and JEDEC (GTL) standards. Figure 1 shows the backplane
model with all four boards connected.
V
TT
R
2 in. 2 in.
Receiver 1 Driver 2
2 in.
Receiver 3
2 in.
Receiver 4
V
TT
R
4 in. 4 in. 4 in.
3-Bit Bus
Figure 1. Backplane Model With All Four Boards Connected
Another design has been used to simulate the transmission-environment effect when transferring data across a longer
point-to-point transmission line. Figure 2 shows the same backplane model with only one driver and one receiver used to transfer
the data across 12-in., 28-in., and 48-in. transmission lines.
Driver
2 in.
Receiver
V
TT
R
12-in., 28-in., or 48-in. Transmission Line
Figure 2. Point-to-Point Model With Only One Driver and One Receiver Connected