Logic Level Signals

TTL or CMOS signals to/from ICs can only be used within the confines of the chassis.

Do This

Best practice is to use optical isolators. This is not as simple as it sounds because you may not want to share the same power supply as the digital signals. At a minimum, insert a series resistor from the PS output to the outside world to limit current if the line were to short.

Second best practice, have external lines actively pulled up. The external switch provides a contact to ground. The pullup should be on the order of 10K. Also have a small series resistor (100 Ohms?) to be a current limit.

Don't

• use logic level signals for switches located outside the chassis.
  There are several problems with this.
  The impedances are moderate (not low), this results in poor noise immunity. The inputs are not robust and any voltage higher than Vcc on the input will be directly put onto the Vcc rail, causing anxiety and possibly smoke.