Datasheet

SCEA017
7–292 GTLP in BTL Applications
As shown in Figure 7, the high-drive SN74GTLPH1655DGGR is used on the driver and receiver
cards. There is a removable termination card at either end of the backplane to allow
performance with different values of R
TT
to be examined. Two cards (driver and receiver) were
modified to operate at BTL signal levels by changing the R/2R V
REF
resistor network to R/3R.
V
TT
for the entire backplane was changed from 1.5 V to 2.1 V by changing the resistor in the
1.5-V V
TT
regulator that sets the output voltage.
Driver Daughter Card
Group-1 Termination Card
(Rear of Backplane)
GTLP Monitor Points
Termination
Resistors
and
Bypass
Capacitors
Connector
Mounting
Pins
SN74GTLPH1655DGGR
AMP Through-Hole Connector
V
REF
Resistors and
Bypass Capacitor
Source-Synchronous
Clock Selection
SN74GTLP1394DGVR
LVTTL
Monitor Points
Slow/Fast
Edge-Rate
Selection
#1 Bit
Not Switch
Selection
Figure 7. GTLP EVM Driver and Termination Cards
Waveforms using the GTLPH1655 device at both normal GTLP signal levels and BTL signal
levels in the fully loaded 17.9-in.-long, 20-slot backplane at various terminations and frequencies
are shown in Figures 8 and 9. The GTLP2033 has the same B-port outputs as the GTLPH1655
and other high-drive GTLP devices, so performance is similar. The actual data frequency of the
waveform is shown, and is one-half the actual clock frequency. Waveforms were taken at the
backplane connector pin. The performance of the GTLP device is excellent at both signal levels.
An R
TT
of 33 Ω most closely matches the loaded-bus line impedance as seen by the
well-behaved incident wave. Other high-drive GTLP devices are expected to operate in a similar
manner at BTL signal levels.