Trying to work on Raspberry Pi 3, Model B V1.2 with several serial ports with C++ code. Previously I should say that when I use only one ports everything works like a charm, so ports configuration is correct. Termios is used with appropriate speeds, stop bits and so on. Timings are configured by

tty.c_cc[VMIN]   =  0;              
tty.c_cc[VTIME]  =  5;                  

My program uses vector of pointers on my own serial port class. In the loop I ask every port from vector and if timing (0.5 s) is over, program continues. If only one serial port is pushed pack, everything is OK. When I add at least one more port, looks like that timings become broken and 0.5 seconds transform to something much more faster and no one byte could be read. I tested my program with several ports on PC with 4.15.0-46-generic #49-Ubuntu and there was not any problem, so I decided that the root of my issue is on Raspberry (or Raspbian). What can I do to find correct workaround?

Short version of my code:

for (tx_uart* port: uarts) {
    // Work with port while getting info
    while (frame_asked.at(port->getID())) {
         bool processed = false;
         // Read buffer while first zero byte
         while (!processed) {
             uint8_t buffer[MY_BUFFER_SIZE];
             size_t readed = port->readBytes(buffer, MY_BUFFER_SIZE);
             if (readed == 0) {
                 std::cout << "nothing" << std::endl;
         // Reading until whole pkg got
    // Processing

So problem is occured when I try to ready bytes in

size_t readed = port->readBytes(buffer, MY_BUFFER_SIZE);

all timing are skipping and always I get readed = 0.

Edit 1: I use serial ports with /dev/ttyUSB* and USB-RS232 convertors.

  • You have given too little information.
    – joan
    Commented Mar 20, 2019 at 8:25
  • @joan I've added sample of my code. What additional info I could provide to make issue more clear?
    – Stanislav
    Commented Mar 20, 2019 at 8:58
  • The Pi has only one usable UART. So you can only have one UART based serial link.
    – joan
    Commented Mar 20, 2019 at 9:10
  • 1
    @joan That is info that I really forgot to add: I use serial port with /dev/ttyUSB* and USB-RS232 convertors.
    – Stanislav
    Commented Mar 20, 2019 at 9:22

1 Answer 1


The issue was more hardware than software. I have connected four USB-RS232 converters and devices connected by this converters to main Raspberry. All devices were powered only through the Raspberry's USB-ports within the converters. So seems to be that there was not enough power to sustain the correct baudrate. Separate powering has solved my issue.

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