Phototransistor feeding into PLC TTL +5 input?

You didn't say how you connected it. There's a few ways. I would wire it as a common-collector with a large resistor at the emitter. By increasing the size of the resistor, you'll get a larger voltage. I don't know the specs of the PLC, but it may have some input impedance which will lower the impedance seen by the emitter, lowering the voltage. In this case, you could build a non-inverting op-amp to boost the amplitude, or use a comparator to flip it's output at a fairly small voltage.