cmos pull up resistor
This discussion assumes 5 V Vcc.
TTL output levels must only drive 2.0 volts or greater for a logic high level. In addition, many TTL drivers source less current than they sink, meaning that while they may be rated for 4 mA drive for logic high, they might be rated at 12 mA for a logic low. As the output voltage increases, the drive current drops to where it may only drive 0.5 mA at 3.5 V.
The pull-up resistor sources additional current to the signal, helping the output driver to get to the 3.5 V CMOS logic high level. This helps meet the logic high CMOS requirement without hurting the logic low level as the TTL output can easily overcome the effects of the pull-up resistor.
In addition to ensuring that the TTL logic high output signal reaches CMOS logic high levels, the pull-up resistor will provide a faster rise time.
I hope this helps answer your question.