No wonder that a simple voltage does not work. Any TTL input needs a rather low line impedance to be tied to low level, or, in other words, it must be tied to GND as good as possible to detect a low level.
An opto-isolator or optocoupler is a good solution since it also protects the inputs from overvoltage. You can also use a simple transistor but make sure that the output never goes above 5V even if the transistor burns out, you can use a 4.7V zener diode across the output of the transistor.
