TTL, RS232, and RS485 Voltage Standards

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TTL, RS232, and RS485 are representations of logic levels, which are three different voltage standards for serial communication.

TTL

TTL: Full duplex (5V system, logic 1: 2.4V–5V, logic 0: 0V–0.5V). TTL refers to Transistor-Transistor Logic circuits. Many “USB to TTL” modules available on the market are actually “USB to TTL level serial” modules. In this case, a signal of 0 corresponds to 0V, and 1 corresponds to either 3.3V or 5V, which is compatible with the I/O levels of microcontrollers and SoCs. When we perform serial communication, the signals coming directly from the microcontroller are generally TTL levels.

RS232

RS232: Full duplex (logic 1: -15V–5V, logic 0: +3V–+15V). This is an asynchronous transmission standard interface established by the Electronic Industries Association, which corresponds to both voltage standards and communication protocols (timing). The voltage standard is: +3V to +15V corresponds to “0”, and -3V to -15V corresponds to “1”. The logic levels of RS232 differ from TTL, but the protocol is the same.

RS485

RS485: Half duplex (logic 1: +2V–+6V, logic 0: -6V–2V). Here, the voltage refers to the voltage difference between the two lines A and B. RS485 is a serial interface standard that uses differential signaling for long-distance transmission. It transmits differential signals, meaning the voltage difference between the two lines A and B serves as the level signal. Differential signals can effectively resist external interference because the interference affects both lines equally, keeping the voltage difference constant and ensuring the signal transmission is unaffected. Unlike TTL and RS232, which can only connect one-to-one, RS-485 allows for up to 128 transceivers to be connected on the bus.

TTL, RS232, and RS485 Voltage Standards

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