Datasheet

SCEA019 - January 2001
7–90 Texas Instruments GTLP Frequently Asked Questions
10 How should I terminate GTL devices in short-distance applications?
GTL products are open drain, so they must be connected to V
TT
via proper R
TT
termination
resistors. One question is that if GTL or GTLP is used to transmit signals on a daughter card,
the distance is very short, about 4 cm to 6 cm. Do we need to position a R
TT
termination
resistor at both the driver and receiver? If so, can you provide the R
TT
based on your
experience? In such a condition, how can better results be obtained?
You need only to put the R
TT
termination resistor at the receiver end of the transmission line.
On the backplanes we recommend putting the termination resistors on both ends of the
backplane because you do not know when slot 1 or 20 will be the transmitter or receiver, so
you have to cover both cases. The R
TT
value should equal the loaded-trace impedance
whether or not you use one or two R
TT
termination resistors. If two are used, the effective
resistance is reduced by a factor of two (i.e., 50 at either end, the driver will see 25-
effective resistance) which means simply that the driver has to be able to sink more current.
For this short trace and lower loading you could probably use a single termination resistor at
the receiver that is about 10 less than the natural trace impedance (i.e., If Z
O
is 50 , use
40 .). Using the GTLP EVM, we have removed the driver termination and the signal integrity
was satisfactory, however, when the receiver termination was removed and only the driver
side was terminated, signal integrity was poor. As you would expect, it did not work when both
terminations were removed.
11 What is the advantage of using GTLP in my backplane?
Increasing bandwidth and data throughput speeds are major issues today for many systems,
as our world becomes increasingly interconnected. TI has taken what it has learned about the
use of GTL over the last four years and applied this expertise and knowledge to future
backplane-based applications using GTLP devices.
TIs GTLP family has been designed specifically for optimum signal integrity in multipoint,
heavily loaded, distributed-capacitance backplanes like those shown in Figure 10. These
improvements allow higher data throughput in the same bit-width backplanes that are
currently being used and provide an easy migration from current TTL/LVTTL and BTL/FB+
backplane drivers to GTLP devices.