TTL Logic Level

A logic level is the voltage range a digital line uses to represent a binary 1 or 0. TTL logic level refers to the family of conventions descended from transistor-transistor logic, and in practice it is shorthand for the voltage you must match when you connect to a board's raw serial pins. Getting it wrong is one of the easiest ways to damage hardware during a repair.

Common voltage levels

Classic 5V TTL treats roughly 0 to 0.8V as a logic low and about 2V up to 5V as a logic high. Most modern embedded boards, including ASIC miner control boards, instead run at 3.3V CMOS levels, and some newer parts use 1.8V. The labels are often used loosely, so people say "TTL serial" to mean a logic-level UART regardless of whether the rail is 5V or 3.3V. What matters is the actual rail voltage on the pins in front of you.

Why matching levels matters

If you feed a 5V signal into a pin expecting 3.3V, you can push current into the chip's protection diodes or exceed its maximum rating and quietly destroy it. The reverse mismatch is gentler but unreliable, because a 3.3V high may not rise far enough for a 5V input to register it as a one. When the two sides differ, a level shifter translates between them safely. Before connecting any USB-to-serial adapter to a board, set the adapter's jumper to the board's rail and measure the pins if you are unsure.

These levels are the raw, on-board form of signalling; for the longer-distance variant with different voltages, see how a serial console distinguishes TTL from RS-232.