Communication between 2 Bela Mini

USB-A to USB-A ... In USB 2.0 you always need one host and one device. You may make it work USB-A to USB-A because you could configure it in software one as a host and one as a device, in defiance of what the connector would suggest ... but then you'd need to find a rare USB-A plug to USB-A plug cable with data lines ...

FelixDoyon Or is it just a basic Serial communication?

If you've set up the IPs, then use the network interface , sending UDP messages back and forth. Use the UdpClient / UdpServer classes.

Something like

// to send
#include <libraries/UdpClient/UdpClient.h>

UdpClient client;

// in setup() 
client.setup(port, ip_address);

// from a non-realtime thread:
client.send(buf, size);

// to receive
#include <libraries/UdpServer/UdpServer.h>

UdpServer server;

// in setup()
server.setup(port);

// from a non-realtime thread:
uint8_t buf[maxLen];
int ret = server.waitUntilReady(timeout)
if (-1 == ret) {
  // handle error
} else if (1 == ret) {
  // receive one packet
  int len = server.read(buf, maxLen false);
  if(len < 0) {
    // handle error
  } else {
     printf("From %s:%d received %d bytes: ", server.getLastRecvAddr(), server.getLastRecvPort(), len);
     for(int n = 0; n < len; ++n)
       printf("%d", buf[len]);
     printf("\n");
  }
}

OK try the following:

• make a backup of your SD card and/or projects
• boot Bela, access it via ssh and edit file /opt/Bela/bela_gadget.sh, replace the line
  ls /sys/class/udc/ > UDC with ls /sys/class/udc/ | head -n1 > UDC
• power off Bela, remove the SD card and place it in your laptop
• the BELABOOT partition appears on your computer, open the file uEnv.txt in a text editor (e.g.: sublime, vim, nano, whatever, don't use Word or TextEdit).
• after the line that starts with uboot_overlay_addr3=, add another line: uboot_overlay_addr4=/lib/firmware/PB-USB1-PERIPHERAL.dtbo
• save the file and quit the editor. Use the OS's "safely remove" feature before you unplug the SD card.
• put the SD card on Bela and boot it, connected only with the USB micro cable
• access Bela via ssh
• create file /opt/Bela/usb1_network.sh with the following content:

  #!/bin/bash
  
  HOST_OS=mac # use 'mac' for ncm or 'win' for RNDIS
  
  cd /sys/kernel/config/usb_gadget/
  mkdir g2 && cd g2
  
  echo 0x1d6b > idVendor  # Linux Foundation
  echo 0x0104 > idProduct # Multifunction Composite Gadget
  echo 0x0102 > bcdDevice # v1.0.2
  echo 0x0200 > bcdUSB    # USB 2.0
  
  mkdir -p strings/0x409
  echo "__bela2__" > strings/0x409/serialnumber
  echo "Augmented Instruments Ltd" > strings/0x409/manufacturer
  echo "Bela2" > strings/0x409/product
  
  # Two USB-network drivers: one for Windows, one for macOS, only one will be enabled below
  FUNC_RNDIS=functions/rndis.usb0
  mkdir -p $FUNC_RNDIS                    # network, Windows compatible
  mkdir -p functions/ncm.usb0             # network, macOS compatible
  
  mkdir -p os_desc
  echo 1 > os_desc/use
  echo 0xbc > os_desc/b_vendor_code
  echo MSFT100 > os_desc/qw_sign
  
  /opt/Bela/bela_mac.sh || true
  # using cpsw_0 and cpsw_3 that are unused by /opt/Bela/bela_gadget.sh
  if [ $HOST_OS == win ]; then
  	cat /etc/cpsw_0_mac > $FUNC_RNDIS/host_addr || true
  	cat /etc/cpsw_3_mac > $FUNC_RNDIS/dev_addr || true
  else
  	cat /etc/cpsw_0_mac > functions/ncm.usb0/host_addr || true
  	cat /etc/cpsw_3_mac > functions/ncm.usb0/dev_addr || true
  fi
  
  mkdir -p $FUNC_RNDIS/os_desc/interface.rndis
  echo RNDIS > $FUNC_RNDIS/os_desc/interface.rndis/compatible_id
  echo 5162001 > $FUNC_RNDIS/os_desc/interface.rndis/sub_compatible_id
  # The below are needed starting from kernel 4.14: the defaults have reverted to 2/6/0,
  # which no longer works with Windows or HoRNDIS
  echo ef > $FUNC_RNDIS/class
  echo 04 > $FUNC_RNDIS/subclass
  echo 01 > $FUNC_RNDIS/protocol
  
  mkdir -p configs/c.1
  echo 500 > configs/c.1/MaxPower
  if [ $HOST_OS == win ]; then
  	ln -s $FUNC_RNDIS configs/c.1/ # this needs to be loaded first in order to work on Windows
  else
  	ln -s functions/ncm.usb0 configs/c.1/
  fi
  
  udevadm settle -t 5 || :
  echo musb-hdrc.1 > UDC
  
  echo "" > /var/lib/dhcp/dhcpd.leases

• if you are using Windows, replace HOST_OS=mac with HOST_OS=win. Either should work for Linux
• make it executable: chmod +x /opt/Bela/usb1_network.sh
• edit file /etc/dhcp/dhcpd.conf and add these three lines close to the lines that do the same for 192.168.7.0 and 192.168.6.0

  subnet 192.168.8.0 netmask 255.255.255.0 {
          range 192.168.8.1;
  }

• edit file /etc/network/interfaces and at the bottom of the file add:

  auto usb2
  iface usb2 inet static
      address 192.168.8.2
      netmask 255.255.255.0
      network 192.168.8.0

• reboot Bela, connected only with the USB micro cable.
• get back onto Bela and execute /opt/Bela/usb1_network.sh
• run ifup usb2
• run dmesg -w and leave it running in the terminal
• moment of truth: connect the USB-A to USB-A cable to the same computer where you are currently connected over micro USB
  • does a new network show up on the host?
  • does anything appear in the terminal window where the process is running?
  • can you connect to the Bela IDE on 192.168.8.2?

Even if this is successful, do not yet connect the two USB cables to two different computers, get back for more details.