了解Linux中的RS-485

Question

I'm trying to learn the RS-485 interface by sending data between two Linux computers. The main computer is running ubuntu16.04 and the other is running a debian based system on kernel 4.19.

The main PC has a USB to RS-485 adapter and the other PC has RS-485 interface.

Initially I assumed that i could simply echo data on the interface but I could not. If i connected with picocom I could type data but not send it. By looking around i realized that this is because both PCs are waiting for RTS that somehow needs to be toggled.

I then concluded that I need some sort of C program to initialize the interface(please correct me if I'm wrong).

I went https://www.kernel.org/doc/Documentation/serial/serial-rs485.txt Where the is an example code for setting up RS-485.

However, when I try to run it always fails at:

status = ioctl(fd,TIOCSRS485, &ctrl485);
  if (status) {
    printf("ERROR PORT 1! TIOCSERSETRS485 failed %i", status);
    return -1;
  }

I have googled every combination of RS-485 that i can think of but most information is about modbus and sensors. I also read about ioctls but I'm still stuck.

If anyone has experience or tips I would be really thankful.

Best Regards, W

Answer 1

I cannot be completely sure but I don't think you need to play with the RTS toggling at all since your devices seem to support hardware half-duplex by default.

Check the following to make sure your hardware setup is correct:

According to the manual of your Cincoze you have to select each port's operating mode at BIOS level:

Then make sure your USB cable is also working half-duplex with the microswitches (it should be fine by default):

And finally, make sure you're wiring your devices correctly, following these diagrams:

If your cable is not too long (less than 50 meters, maybe?) you should not have to worry about termination or impedance that much, otherwise you might need to add a resistance on the computer DB9 and move the switches on the USB cable.

As a final comment, be aware that your devices support full-duplex too; if you change the settings as per the instructions above but select RS422 instead and make the 4-wire setup you should be able to open a terminal on both computers and send and receive at the same time, in case you need that.

EDIT 1: For completeness, I'm adding here a couple of references for those who are working with devices that don't support hardware direction control:

RS485: Inappropriate ioctl for device

Pymodbus - Read input register of Energy meter over rs485 on uart of raspberry pi3

EDIT 2: As discussed in the comments below, for this particular hardware where you can choose different operating modes in the BIOS, an additional thing to keep in mind is that the naming of the ports (at OS level) might change when you select a new operating mode. Use $cat /proc/tty/driver/serial to find out details about you hardware.