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I am trying to debug an LCD device with an RS232 UART interface.

I am using RealTerm on a Windows 10 machine.

The UART interface works properly. I have checked the sending device, using an oscilloscope (see below). On power-up, the device sends a hexidecimal sequence "5A A5..." to the UART interface.

enter image description here

However, when using RealTerm, I am not able to get this data on startup, or any other data. I am using a FTDI TTL 5V cable, as recommended.

I have checked if the TX/RX wires are mixed up. This part is OK, I think. (Connected output of LCD to yellow input wire of FTDI cable.)

Voltage between red and black wire is 5V, measured with a multimeter.

The settings in RealTerm are OK, I think. I use baudrate 115200, 8N1, which agrees with the datasheet. I have also checked the settings with the company of the LCD. On startup, I see BREAK and Error becoming red in the status panel.

I am debugging by connecting and disconnecting the power wires between the UART and the FTDI cable.

I don't know in which direction I should look further to debug this.

I haven't installed anything from FTDI (like a dll driver) yet on my computer. Maybe that's the issue?

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    \$\begingroup\$ you need the driver otherwise you can't select the virtual com port you are actually connected to in realterm (or any terminal software) \$\endgroup\$
    – DKNguyen
    Commented Aug 6, 2020 at 15:08
  • \$\begingroup\$ DLL file is not going to help your cause. You need to look up the FTDI driver for your UART adapter. \$\endgroup\$
    – user103380
    Commented Aug 6, 2020 at 16:04
  • \$\begingroup\$ Which LCD that is? Link to datasheet which explains the interface? That is also definitely not 5V TTL UART data, the signal idles low while UART will idle high. That is RS232 level data. \$\endgroup\$
    – Justme
    Commented Aug 6, 2020 at 18:11
  • \$\begingroup\$ The LCD has model nr. TY068HWXH11 from the company Shanghai Top Display Optoelectronics (STDO). However, I could not find an online datasheet at the moment. \$\endgroup\$
    – user270124
    Commented Aug 7, 2020 at 13:45

1 Answer 1

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The display outputs 5V/0V signal but with RS232 compatible polarity, for direct connection into a RS232 input. The FTDI cable has 5V TTL input, so it does not have RS232 input. You need a different USB cable with RS232 interface, or you need a RS232 tranceiver to convert to TTL levels, or maybe an inverter to just invert the polarity.

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