Noise on serial line

[CarbonMan]

I am connecting 2 esp32s together, communication is one-way. The sender is a Makerfab esp32uwb based on the WROOM and the receiver is a WT32-SC01 based on the WROVER.

The sender initializes the connection with

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1. Serial1.begin(9600, SERIAL_8N1, RXD1, TXD1);

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where TXD1 is 12

The receiver initializes the connection with

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1. Serial1.begin(9600, SERIAL_8N1, RXD1, TXD1);

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where RXD1 is 13
Grounds are connected.

The connection works, I can send data. However, there is constant noise on the receiver.
I have tried both

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1. pinMode(RXD1, INPUT_PULLUP);

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and

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1. pinMode(RXD1, INPUT_PULLDOWN);

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on the receiver, but it makes no difference.

I have tried different pins on the sender and the receiver.
Interestingly this did not happen when I used a different board (it was a DEVKIT V1 DOIT I believe) as the sender.

Edit: I have tried changing the sender and receiver to

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1. SERIAL_7E2

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but it didn't help.

Any ideas?

[username]

How long is the serial cable.

[CarbonMan]

Very short. Maybe 10cm.