A binary logic table translates abstract digital states (1s and 0s) into physical voltage boundaries. While a truth table tells you what a logic gate does, a binary logic voltage specification table tells you how to drive it without causing undefined states, oscillation, or silicon damage. Whether you are interfacing a 3.3V ESP32 to a 5V relay board or debugging a noisy industrial sensor line, knowing the exact $V_{IH}$, $V_{IL}$, $V_{OH}$, and $V_{OL}$ thresholds for your specific logic family is the difference between a reliable circuit and a bench headache.
The Master Binary Logic Voltage Table
The table below consolidates the DC voltage thresholds for the most common logic families used in hobbyist, educational, and light industrial applications. Data is sourced from Texas Instruments SN74 series datasheets and standard JEDEC specifications for LVCMOS/LVTTL interfaces.
- $V_{CC}$: The nominal supply voltage for the IC.
- $V_{IL}$ (Max): The highest voltage the chip will reliably read as a Logic 0. Anything below this is safe.
- $V_{IH}$ (Min): The lowest voltage the chip will reliably read as a Logic 1. Anything above this is safe.
- $V_{OL}$ (Max): The highest voltage the chip will output when driving a Logic 0 (under rated load).
- $V_{OH}$ (Min): The lowest voltage the chip will output when driving a Logic 1 (under rated load).
- Noise Margin: The calculated buffer zone ($V_{OH} - V_{IH}$ for High; $V_{IL} - V_{OL}$ for Low) that protects against EMI.
| Logic Family | $V_{CC}$ | $V_{IL}$ (Max) | $V_{IH}$ (Min) | $V_{OL}$ (Max) | $V_{OH}$ (Min) | Noise Margin (Low/High) |
|---|---|---|---|---|---|---|
| 74LS (TTL) | 5.0V | 0.8V | 2.0V | 0.4V | 2.7V | 0.4V / 0.7V |
| 74HC (CMOS) | 5.0V | 1.35V | 3.15V | 0.1V | 4.4V | 1.25V / 1.25V |
| 74HCT (TTL-compat) | 5.0V | 0.8V | 2.0V | 0.1V | 4.4V | 0.7V / 2.4V |
| 74HC (CMOS) | 3.3V | 0.9V | 2.31V | 0.1V | 2.9V | 0.8V / 0.59V |
| 74LVC (Low-V CMOS) | 3.3V | 0.8V | 2.0V | 0.4V | 2.4V | 0.4V / 0.4V |
| CD4000B (Std CMOS) | 5.0V | 1.5V | 3.5V | 0.05V | 4.95V | 1.45V / 1.45V |
Source: JEDEC JESD8C LVCMOS standards and All About Circuits logic level analysis. Values assume standard $I_{OL}$/$I_{OH}$ test loads (typically 4mA to 8mA) at 25°C ambient.
Which Column Applies to Your Installation?
The most common mistake makers and junior engineers make is looking at the 5V row when their microcontroller operates at 3.3V. The column that applies to your circuit is strictly dictated by the $V_{CC}$ pin of the receiving integrated circuit.
If you are connecting a 3.3V ESP32 GPIO to a 5V 74HC logic gate, you must look at the 74HC 5.0V row for the receiving chip. The ESP32 outputs a maximum of 3.3V ($V_{OH}$). However, the 74HC chip requires a minimum of 3.15V ($V_{IH}$) to register a Logic 1. While 3.3V technically exceeds 3.15V, your noise margin is a razor-thin 0.15V. Any voltage drop across a breadboard trace or slight ground bounce will push the signal into the undefined region (between 1.35V and 3.15V), causing the gate to oscillate or read as a 0.
Derating Rows and What the Table Hides
How Derating Modifies the Base Value
The values in the master table are measured under ideal bench conditions: 25°C ambient temperature and minimal capacitive loading. In practice, derating factors modify these baseline thresholds:
- Temperature Derating: As silicon heats up, carrier mobility drops. For standard CMOS, $V_{OH}$ will decrease and $V_{OL}$ will increase slightly at 85°C, shrinking your noise margin by roughly 10% to 15%.
- Current Load Derating: The $V_{OH}$ and $V_{OL}$ specifications assume a specific output current (e.g., $I_{OH} = -4mA$). If you attempt to source 20mA to drive an LED directly from a logic pin, the internal resistance of the MOSFETs will cause $V_{OH}$ to sag well below the guaranteed 4.4V minimum, potentially failing to trigger the next stage.
- Capacitive Loading: Driving long ribbon cables or multiple gate inputs increases parasitic capacitance. This does not change the DC voltage thresholds, but it severely degrades the edge rate (rise and fall times), leaving the signal lingering in the undefined linear region where shoot-through current spikes and EMI susceptibility peak.
What the Binary Logic Table Cannot Tell You
A voltage threshold table is a static DC reference. It is completely blind to dynamic AC characteristics. You cannot use this table to determine:
- Propagation Delay ($t_{pd}$): The time it takes for an input change to reflect at the output. A CD4000B gate might have excellent noise margins, but its $t_{pd}$ at 5V is roughly 125ns, making it useless for SPI buses running above 2MHz.
- Setup and Hold Times: For flip-flops and latches, the timing windows required around the clock edge to prevent metastability.
- Dynamic Power Consumption: CMOS chips draw near-zero static current, but every logic transition charges and discharges internal capacitance, drawing transient current spikes that scale linearly with switching frequency.
Binary Logic Table FAQ
How do I use a binary logic table to interface 3.3V and 5V devices safely?
Look at the output specifications ($V_{OH}$, $V_{OL}$) of the transmitting device and compare them to the input requirements ($V_{IH}$, $V_{IL}$) of the receiving device. If the 3.3V transmitter's $V_{OH}$ (typically 2.9V to 3.1V under load) is lower than the 5V receiver's $V_{IH}$ (often 3.15V or 3.5V), you have a logic level mismatch. You must insert a level shifter (like the TXS0108E or a simple 74LVC245) or use a receiver family with TTL-compatible inputs (74HCT) to bridge the gap without risking undefined logic states.
Why does my 74HC chip read a 3.3V signal as a logic 0 when powered at 5V?
This is the most common trap in mixed-voltage digital design. When a 74HC chip is powered at 5V, its $V_{IH}$ (minimum voltage to read a 1) is typically 70% of $V_{CC}$, which equals 3.5V. If your ESP32 or Raspberry Pi outputs 3.3V, it falls short of the 3.5V threshold. The 74HC chip sees the 3.3V signal as being in the undefined region or as a Logic 0. Swap the 74HC chip for a 74HCT chip, which is specifically designed with a lower $V_{IH}$ of 2.0V to accept 3.3V and older TTL logic levels while running on a 5V supply.
Does a binary logic truth table show current draw or fan-out limits?
No. A binary logic truth table only maps boolean inputs to boolean outputs (e.g., A=1, B=0 yields Y=0). To find current draw and fan-out limits, you must consult the DC electrical characteristics section of the component's datasheet. Fan-out is calculated by dividing the maximum output current ($I_{OH}$ or $I_{OL}$) of the driving gate by the maximum input leakage current ($I_{IH}$ or $I_{IL}$) of the receiving gates. For modern CMOS families like 74HC, input leakage is in the nanoamp range, meaning DC fan-out is virtually limitless; the real limit becomes AC fan-out, dictated by parasitic capacitance and acceptable propagation delay degradation.
