# FT232R TTL Logic Levels

I have an FT232R chip hooked up to my oscilloscope with the TX and RX pins joined. I'm sending characters directly to it via Putty. The logic levels I am seeing are 5V low and 0V high. I thought the output of this chip was TTL (i.e. 0V low and 5V high)? Am I missing something?

• In UART communication, the idle, no data state is HIGH. The start bit is logic LOW, followed by multiple bits of data, and a HIGH stop bit. Is it consistent with what you are seeing? en.wikipedia.org/wiki/… – 比尔盖子 Jun 24 '19 at 19:25
• 0V and 5V are TTL levels ... you are missing the fact that logic signals have active levels ... for example an active low means that 0V = logic 1 – jsotola Jun 24 '19 at 19:30
• @比尔盖子 - Absolutely correct. For some reason I assumed that no data state would be LOW. +1 for an interesting fact about the historical legacy relating to telegraphy. – Joe Mann Jun 24 '19 at 19:31

UART communication uses positive logic. In TTL, logic 1 is 5V, logic 0 is 0V. However, in UART, the idle, no data state is 1/HIGH, not 0/LOW. What you are seeing here is not inverted logic level, but simply an idle UART line.
For completeness, here's an example. I'm sending ASCII character "U" (01010101) via UART, 1 start bit, 8-bit data, no parity, 1 stop bit, 9600 baud.
On an oscilloscope, you will see the following waveform. Because the least significant data bit is transmitted first, binary digits of ASCII U is reversed.