Differences Between TTL and RS232 LevelsWhat are TTL levels, CMOS levels, and RS232 levels? What are their differences? Generally speaking, CMOS levels have a higher noise tolerance than TTL levels.(1) TTL Level StandardsOutput L: <0.8V; H: >2.4V.Input L: <1.2V; H: >2.0VThe output low level of TTL devices must be less than 0.8V, and the high level must be greater than 2.4V. For input, below 1.2V is considered 0, and above 2.0 is considered 1. Thus, the noise tolerance for the low input level of TTL is only (0.8-0)/2=0.4V, and the noise tolerance for the high level is (5-2.4)/2=1.3V.(2) CMOS Level StandardsOutput L: <0.1*Vcc; H: >0.9*Vcc.Input L: <0.3*Vcc; H: >0.7*Vcc.Since the CMOS power supply is 12V, an input below 3.6V is considered low, with a noise tolerance of 1.8V, and above 3.5V is considered high, also with a noise tolerance of 1.8V. This is higher than TTL.(3) RS232 StandardThe level of logic 1 is -3 to -15V, and the level of logic 0 is +3 to +15V, note that the level definition is inverted.What are the differences between using TTL and CMOS levels?1. The definitions of the upper and lower limits of levels are different; CMOS has a larger noise immunity area. With the same 5V supply, TTL is generally around 1.7V and 3.5V, while CMOS is generally around 2.2V and 2.9V; this is not accurate and is for reference only.2. The current driving capability is different; TTL generally provides 25mA of driving capability, while CMOS is generally around 10mA.3. The required input current also differs; TTL generally needs around 2.5mA, while CMOS almost requires no input current.4. Many devices are compatible with both TTL and CMOS; the datasheet will provide details. If speed and performance are not a concern, devices can generally be interchanged. However, it should be noted that sometimes load effects may cause circuit operation issues, as some TTL circuits require the input impedance of the next stage as a load to function properly.1. Logic levels of TTL and CMOS circuitsVOH: Output voltage of logic level 1VOL: Output voltage of logic level 0VIH: Input voltage of logic level 1VIL: Input voltage of logic level 0Critical values for TTL circuits:VOHmin = 2.4V VOLmax = 0.4VVIHmin = 2.0V VILmax = 0.8VCritical values for CMOS circuits (with a power supply voltage of +5V)VOHmin = 4.99V VOLmax = 0.01VVIHmin = 3.5V VILmax = 1.5V2. Logic Level Conversion Between TTL and CMOSCMOS levels can drive TTL levelsTTL levels cannot drive CMOS levels and require a pull-up resistor.3. Features of Common Logic Chips74LS Series: TTL Input: TTL Output: TTL74HC Series: CMOS Input: CMOS Output: CMOS74HCT Series: CMOS Input: TTL Output: CMOSCD4000 Series: CMOS Input: CMOS Output: CMOS——————————————————————————————————Diagram: Inversion of RS232 and TTL/CMOS levels.
