Datasheet

SCEA019 - January 2001
7–84 Texas Instruments GTLP Frequently Asked Questions
All three devices shown in Figure 4 have bus hold on the LVTTL A port. But, only the newer TI
GTLP family uses H in the part number to denote bus hold, because there are several devices
in the GTLP family without bus hold. All TI GTL devices have bus hold, while the competitor’s
GTL or GTLP family has some of both; the only way to determine the configuration is by
referring to the data sheet.
A typical single-bit distributed-capacitance open-drain/open-collector backplane physical
representation is shown in Figure 5. Normally, backplanes are 4, 16, 32, 64, 128, or more bits
wide, with each bit identical to the representation in Figure 5. Both GTL and GTLP devices
need a pullup resistor to pull the bus high. In this representation, the device in slot 1 is on,
pulling the trace below 0.55 V, and providing a valid low. Receivers in slots 2 through 20
detect that the signal is low and, if not in the high-impedance state, transmit a low logic level
to the A-port daughter-card side. Although not shown, each driver/receiver on the daughter
card is a transceiver that can be configured to transmit or receive signals from the backplane.
When the device in slot 1 is turned off, the bit is pulled high to 1.5-V by the 22-Ω termination
resistors. The receivers in slots 2 through 20 detect that the signal is now high and transmit a
high logic level to the daughter card. This process is repeated multiple times, with any card
being able to transmit low signals onto the backplane. This can be done, even with multiple
devices at the same time (wired-OR), without bus contention. This example assumes true
logic.
Z
O
= 50
GTLP Off
Logic High
GTLP On
Logic Low
Figure 5. GTLP Open-Drain Backplane Physical Representation
GTLP devices are offered in two drive levels to match the termination-resistance value. For best
signal integrity and incident-wave switching, the termination resistor should match or be smaller than
the effective trace impedance. The effective trace impedance and, hence, the optimum termination-
resistance value, varies, based on stub length, device C
io
, slot pitch, and type of connector. Medium-
drive (50 mA) devices are good replacements for existing TTL/LVTTL devices where higher
frequencies are needed, but lower power consumption is a concern. These devices should be used
with termination-resistor values ≥38-Ω. High-drive (100 mA) devices are best for replacing
ABTE/FB+ devices where lower termination resistor values (22 Ω to 38 Ω) are required. Either
medium- or high-drive devices can be used with higher resistance (e.g., 60 Ω) termination resistors,
with the only outcome being the device pulls V
OL
to a lower level, closer to GND, and probably
poorer signal integrity, unless the effective trace impedance also is 60 Ω.