Most laptops and desktops no longer have serial portsserial ports, but many development boards require a serial port for debugging, console interface, or even software download. The serial ports on development boards usually provide “logic level” signals rather than RS-232 serial port signals.
This means that there’s no easy way to connect a development board to a laptop. The USB to TTL converter solves this problem by providing a serial port connection between a host computer and a development board, with the correct interfaces and signal levels for each. Details below.
What is a TTL signal in communication?
A TTL Signal is a kind of hardware interface standard based on the electrical properties of TTL (Transistor-Transistor Logic).
For a TTL input this means that anything below 0.8 volts is a “zero” and anything above 2.4 volts is a “one,” and that it presents a load of less than 1.6ma to the driving circuit.
A TTL output can typically drive ten TTL inputs, and still maintain the correct voltage levels for “zero” and “one.”
Why do we use a USB to TTL converter?
The name USB to TTL converter is a really bad name. It misuses the term TTL, and abuses the term converter. Confusing, right?
The so-called USB to TTL converter is actually a USB Serial Port to CMOS Logic-Level Serial Port converter. It has two ends: a USB connector, and four wires made to attach to terminal posts. It contains a chip that’s able to trick your laptop into thinking a serial port is installed.
You plug the USB port into your laptop, and it appears to your laptop as a new serial port. It uses standard laptop USB serial port device drivers. You connect the four wires up to power, ground, serial RX and serial TX on an Arduino or other development board, these signals use the correct voltage levels for the development board rather than conventional RS-232 levels, and it appears to your Arduino that something is talking to it over its serial port.
More Information
Several manufacturers make this kind of converter; the data sheet for this one does a good job of explaining what the cable is and how it works: USB to TTL Serial Cable - Debug / Console Cable for Raspberry Pi
https://www.quora.com/What-is-a-TTL-signal-in-communication-and-when-using-a-USB-port-why-do-we-use-a-USB-to-TTL-converter
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