15
Mar
2015

Directly connecting two Raspberry Pi’s with serial UART problems

I decided to try directly connecting two Raspberry Pi’s to each other to test out controlling one Pi with the other using simple bash commands that can be sent over UART.

wpid-wp-1426268091384.jpeg

The RX line of RPi 1 (the older Pi with the yellow jack) is connected to the TX line of RPi 2 (the new one with way more GPIO pins) using the black wire, and the opposite is done with the white wire to allow the other direction to talk. My logic analyser is connected using some small wires to the lines so I can monitor the communication. I could see bootup messages on RPi 2 coming from RPi 1, and eventually I got a login prompt. I am using screen /dev/ttyAMA0 to connect to the serial port:

Screenshot - 130315 - 17:38:46

However, there is a really weird problem that I am experiencing. RPi 2 will only send 16 characters before the whole screen session locks up with no way of regaining control. Here is what the logic analyser shows:

Screenshot - 130315 - 17:42:06

Screenshot - 130315 - 17:41:37

The terminal shows the 16 characters that were sent by RPi 2 to RPi 1 in the bottom chart, and the responses from RPi 1 are in the top chart. The decoded protocols at the side also show the 16 numbers I sent. No amount of button mashing gets more characters to display!

It is clearly a problem with the newer RPi 2 – htop running shows loads of data coming back over the UART receive line and also htop is present in the screen session:

Screenshot - 130315 - 17:57:41

Maybe I should try a lower baud rate? It is currently running at 115200, but this doesn’t seem to be a baud rate problem since all the data is getting across just fine, its just that the data stops after 16 characters.

I will try a lower baud rate, watch this space!

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{3 Responses to “Directly connecting two Raspberry Pi’s with serial UART problems”}

  1. Still watching. any news? does this work?

    Behrooz

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