basic connection question - basic work voltage of arduino due

hello everyone,
i am working on a little project where i use a pololu maestro 12 channels servocontroller to contol several servos and a EASYDRIVER STEPPER MOTOR DRIVER to control a stepper motor. Both platform works at a 5V basic voltage, and i know that it might be a problem to connect them to an input pin of my arduino due. The question is, what is the risk of connecting them only to output pins of the arduino and controll them with 3.3V based ttl. (it might not work/ damage the arduino).
again, i am connecting only the tx pin of the arduino to the devices, it is one way communication!
thanks a lot!