What must be done to interface CMOS to TTL?

• A dropping resistor must be used on the CMOS 12 V supply to reduce it to 5 V for the TTL.
• As long as the CMOS supply voltage is 5 V, they can be interfaced; however, the fan-out of the CMOS is limited to two TTL gates.
• A 5 V Zener diode must be placed across the inputs of the TTL gates in order to protect them from the higher output voltages of the CMOS gates.
• The two series cannot be interfaced without the use of special interface buffers designed for that purpose, such as the open-collector buffers.