Datasheet
SCEA019 - January 2001
Texas Instruments GTLP Frequently Asked Questions 7–83
7 What is the difference between GTLP and GTL?
TI introduced the GTL family of devices in 1993. GTL devices were designed with faster edge
rates to drive a lumped load. Originally, these devices were designed for small buses on a
board, e.g., memory applications, and did not support hot insertion. The devices rang
excessively when used in larger multipoint distributed-capacitance backplane applications
because of the faster edge rate. Maximum backplane frequency was limited to about 30 MHz.
GTLP devices were introduced in 1997 to serve distributed-capacitance backplane
applications. GTLP is a subset of GTL devices, with one major difference. GTLP incorporates
improved output edge-control (OEC) circuitry that slows the edge rate and reduces ringing,
which allows maximum possible frequencies above 80 MHz in backplane applications. GTLP
devices can operate at GTL signal levels, but are optimized for, and normally only specified at,
GTL+ signal levels (V
OL
= 0.55 V to V
OH
= 1.5 V, with V
ref
= 1.0 V). GTLP backplane-
optimized devices normally refer to GTLP instead of GTL+ as the 1.5-V V
OH
/1.0-V V
ref
standard, while GTL lumped-load optimized devices refer to GTL+ at this voltage level.
The waveforms in Figure 4 demonstrate the difference between the TI SN74GTL16612 and
the newer SN74GTLPH16612, and how the backplane-optimized edge rates improve signal
integrity in a distributed load.
Test results for the competitor’s GTLP16612, a pin-for-pin functional equivalent of the
SN74GTLPH16612, is shown in Figure 4 for comparison. These receiver waveforms were
obtained using a fully loaded 16-slot demonstration backplane, with Z
O
= 50 Ω, R
TT
= 24 Ω,
slot spacing = 0.9 inch, and data frequency of 37.5 MHz. The driver card was in slot number 8
and the receiver card was in slot number 1.
Figure 4. Demonstration of Output Edge Control
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
-0.000000004 0.000000006 0.000000016 0.000000026
Competitor’s
GTLP16612
TI GTLPH16612
TI GTL16612
Nanoseconds
Volts
0 10 20 30