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I'm trying to track a signal coming to my microcontroller, but I ran into an issue where the USB-UART converter only sends the signal from the PC when the UART is not connected to the microcontroller, meaning the converter's RX and TX are just left floating. When I connect RX and TX to the microcontroller, the signal doesn’t appear.

I’m observing the signal with a logic analyzer.

For sending, I'm using the Terminal program and a USB-TTL converter based on the CH340G.

I’ll describe the issue a bit more specifically. I’m sending a message through Terminal using the CH340G, a USB-TTL chip that isn’t connected to anything, with its pins pulled up to +3.3V. When I connect a logic analyzer to the chip, I can clearly see the signal I’m sending through Terminal. However, when I connect the USB-UART to the microcontroller and send a message via Terminal, no signal is generated; the logic analyzer shows nothing.

What could be the problem?

upd1. Here is an USB-TTL converter schematic (source: https://oshwlab.com/makmak19992542/ch340g-usb-to-rs232-ttl-module-schematic_ks by Kritsana Sudlar). I connect pin4 of CON2 to RX of MCU, pin5 of CON2 to TX of MCU. Pin2 connected to pin 3. GND of USB-TTL conv connected to GND of MCU. USB-TTL converter schematic

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  • \$\begingroup\$ To help get an answer can you edit the question to include a schematic of how the USB-UART converter is connected to the microcontroller, including ground and power supply pins. \$\endgroup\$
    – Chester Gillon
    Commented Oct 28 at 10:17
  • \$\begingroup\$ Many things could be wrong, add details which wires you connected and where. TX, RX and GND. What are the voltages, and do you have an RS-232 cable or TTL/CMOS cable. \$\endgroup\$
    – Justme
    Commented Oct 28 at 10:27
  • \$\begingroup\$ Is the pin on the processor set up as an output pin? \$\endgroup\$
    – DiBosco
    Commented Oct 28 at 11:03
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    \$\begingroup\$ Have you crossed Tx and Rx? (i.e: CH340 Tx -> MCU Rx, and vice-versa) \$\endgroup\$
    – Attie
    Commented Oct 28 at 11:19
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    \$\begingroup\$ What does your MCU board schematic looks like? Many boards already have something driving the MCU RX input, or leave it disconnected for optional use. \$\endgroup\$
    – Justme
    Commented Oct 28 at 12:42

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