Datasheet

SCEA019 - January 2001
Texas Instruments GTLP Frequently Asked Questions 7–85
Incident-wave switching ensures that the first received signal during a transition (low to high or
high to low) is valid, and reduces wait time. Reflected-wave switching requires additional wait
time as shown in Figure 6.
SWITCHING WITH THE
INCIDENT WAVE
SWITCHING WITH THE
REFLECTED WAVE
A⇒B
t
pd Driver
+ t
pd Receiver
= 5 ns + 5 ns = 10 ns
t
pd Driver
+ t
pd Line
+ t
pd Line
+ t
pd Receiver
= 5 ns + 10 ns + 10 ns + 5 ns = 30 ns
A⇒C
t
pd Driver
+ t
pd Line
+ t
pd Receiver
= 5 ns + 10 ns + 5 ns = 20 ns
t
pd Driver
+ t
pd Line
+ t
pd Receiver
= 5 ns + 10 ns + 5 ns = 20 ns
Worst case 20 ns 30 ns
Figure 6. Reflected-Wave Switching
Additional information on incident-wave switching is in the Fast GTLP Backplanes With the
GTLPH1655 application report, literature number SCBA015A.
In summary, GTLP is a bidirectional translator (5-V CMOS, TTL or LVTTL to GTLP, and GTLP
to LVTTL or TTL) that facilitates significantly higher frequencies (>80 MHz) and higher data
throughput in multipoint, heavily loaded, distributed-capacitance backplane applications
because the B-port GTLP output transistors produce these features:
• Reduced voltage swing (<1 V), which reduces EMI over TTL solutions.
• Optimized OEC with low slew rates (0.35 to 0.5 V/ns), which minimizes overshoot in
distributed loads.
• Open-drain configuration with matched termination resistors (≥22 Ω high drive or ≥38 Ω
medium drive), which ensures incident-wave switching and optimum signal integrity.
• I
OFF
, PU3S, and BIAS V
CC
precharge circuitry prevent data disturbance during card
insertion or removal, permitting full live-insertion capability.
L
O
≈ 6 nH/cm
Z
O
= 25 Ω
C
O
≈ 10 pF/cm
τ = 25 ns/m
t
pd
= 5 ns
I = 40 cm
A
B
d = 2 cm
C
t
pd
= 5 ns