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APPLICATION INFORMATION

GTL Background Information

Input Characteristics

GTL Output Characteristics

To Other Inputs

Input

Output

GTL Bus

SN54GTL16612A, SN74GTL16612A

18-BIT LVTTL-TO-GTL+ UNIVERSAL BUS TRANSCEIVERS

SCES187D – JANUARY 1999 – REVISED JULY 2005

GTL was approved as JEDEC standard JESD 8-3 in 1993 and originally was created as a reduced-swing I/O

driver technology to support high-speed buses and backplanes. The GTL bus is designed to work with low

voltage swings. The input buffer works like an analog comparator rather than like an inverter, which allows the

GTL inputs to switch quickly without needing to be driven rail to rail. GTL drivers were designed to pull a 1.2-V

signal down to 0.4 V when switched on. This, however, placed the reference voltage for the input comparator at

0.8 V, which made it susceptible to ground-bounce noise. A variant of GTL, called GTL+, is being used to

address this noise-margin concern. The GTL+ termination voltage is raised to 1.5 V, with the driver pulling down

TI GTL devices operate at, and are specified for, both GTL and the improved-noise-margin GTL+ standard.

However, the 'GTL16612A devices deviate from this history. They are designed with slow rising and falling

edges, to offer significant system frequency improvement in heavily loaded backplanes. They are AC specified

only at GTL+ because most applications are moving to this improved-noise-margin standard; they operate at

either GTL or GTL+.

(improved-noise-margin GTL+ standard). Fast-edge GTL devices are best for point-to-point or lower-frequency

backplanes. Slow-edge GTL devices extend backplane operations to cover even higher frequencies.

The input characteristics are identical on both A and B ports. Both ports are very high impedance and have an

input diode to provide protection against high negative-voltage spikes. The input diode conducts and prevents

more sensitive components from being destroyed as the result of electrostatic discharges or line reflections.

The principle of the GTL bus is based on open-drain drivers, as shown in Figure 3.

Figure 3. GTL Bus: An Open-Drain Bus

match the fully loaded backplane impedance, not the trace impedance, to provide an optimum termination of the

bus and avoid line reflections. The resistance of the GTL output is in the range of a few ohms. However, in the

levels, not to exceed the 'GTL16612A absolute maximum output current of 80 mA, and should be greater than 50

Ω at GTL+ signal levels, not to exceed the recommended output current limit of 34 mA.

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