The SN74GTLPH1655 is a high-drive, 16-bit UBT transceiver that provides LVTTL-to-GTLP and
GTLP-to-LVTTL signal-level translation. It is partitioned as two 8-bit transceivers and allows for transparent,
latched, and clocked modes of data transfer. The device provides a high-speed interface between cards
operating at LVTTL logic levels and a backplane operating at GTLP signal levels. High-speed (about three times
faster than standard LVTTL or TTL) backplane operation is a direct result of GTLP's reduced output swing
(<1 V), reduced input threshold levels, improved differential input, OEC circuitry, and TI-OPc circuitry.
Improved GTLP OEC and TI-OPC circuits minimize bus-settling time and have been designed and tested using
several backplane models. The high drive allows incident-wave switching in heavily loaded backplanes with
equivalent load impedance down to 11 .
GTLP is the Texas Instruments (TI) derivative of the Gunning Transceiver Logic (GTL) JEDEC standard
JESD 8-3. The ac specification of the SN74GTLPH1655 is given only at the preferred higher noise-margin
GTLP, but the user has the flexibility of using this device at either GTL (VTT = 1.2 V and VREF = 0.8 V) or GTLP
(VTT = 1.5 V and VREF = 1 V) signal levels.
Normally, the B port operates at GTLP signal levels. The A-port and control inputs operate at LVTTL logic levels,
but are 5-V tolerant and are compatible with TTL and 5-V CMOS inputs. VREF is the B-port differential input
This device is fully specified for live-insertion applications using Ioff, power-up 3-state, and BIAS VCC. The Ioff
circuitry disables the outputs, preventing damaging current backflow through the device when it is powered
down. The power-up 3-state circuitry places the outputs in the high-impedance state during power up and power
down, which prevents driver conflict. The BIAS VCC circuitry precharges and preconditions the B-port
input/output connections, preventing disturbance of active data on the backplane during card insertion or
removal, and permits true live-insertion capability.
This GTLP device features TI-OPC circuitry, which actively limits overshoot caused by improperly terminated
backplanes, unevenly distributed cards, or empty slots during low-to-high signal transitions. This improves
signal integrity, which allows adequate noise margin to be maintained at higher frequencies.
High-drive GTLP backplane interface devices feature adjustable edge-rate control (ERC). Changing the ERC
input voltage between GND and VCC adjusts the B-port output rise and fall times.This allows the designer to
optimize system data-transfer rate and signal integrity to the backplane load.
Active bus-hold circuitry holds unused or undriven LVTTL data inputs at a valid logic state. Use of pullup or
pulldown resistors with the bus-hold circuitry is not recommended.
When VCC is between 0 and 1.5 V, the device is in the high-impedance state during power up or power down.
However, to ensure the high-impedance state above 1.5 V, the output-enable (OE\) input should be tied to VCC
through a pullup resistor; the minimum value of the resistor is determined by the current-sinking capability of
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