Arduino connecting to a TTL device

Using Arduino Project Guidance
#1

I am new to Arduino. I have figured out the programming to drive a pin to a high voltage to make a fast square wave of about 200 microseconds.

What I want to do is use that pin to control a TTL device. I want to be careful and want to avoid burning up the TTL device with too high of voltage or current from the Arduino. Does anyone have advice on this?

I looked up the general description of a TTL device and it says that it can take an input of 2 V to 5 V for a “HIGH” with a current of 40 uA.

The TTL “LOW” input is described as 0 V to 0.8 V and current of 1.6 mA.

How do I take the HIGH and LOW ouput of the pin of the Arduino and connect it to my TTL device without damaging the TTL device with too much voltage or current from the Arduino?

I am thinking I need to put a resistor (~10k-ish) between the Arduino output pin and ground. But I don’t know what value is best. Then to prevent current from going into the TTL device, I am thinking of putting a much large resistor (~1 Mega-Ohm) and connecting it from the Arduino output pin to the TTL device’s input.

Does this sound good or not? Will it make the Arduino output behave like a TTL input for the TTL device?

I don’t know if this matters, but the Arduino is powered by a USB connection.

#2

Just connect them. The TTL part will only draw the current it needs for a high, and the Arduino will only sink the current required to make a low. No resistor needed.

#3

If you are not sure, the Arduino pins are more sensible than TTL inputs or outputs. But typically a direct connection will work (on not too long lines). You only should avoid connecting TTL outputs directly to an Ardunio powered by 3.3V, in contrast to the 5V supplied to TTL circuits. Only in this case a resistor (1-10k) may protect the Arduino inputs.

BTW TTL inputs usually are tied HIGH, if you want to assure a defined signal level on unconnected pins or when the Arduino is turned off. The TTL HIGH input current is significantly lower than the LOW current, allowing for bigger resistors to Vcc and less power consumption.

#4