FTDI driven USB to RS485 requires a undocumented timeout after write?

Question

I am developing a controller for Mobitec Bus destination panel. The intended communication layer is SBC -> USB 2 RS485 [FTDI] -> Mobitec.

This is also the first time for me working with serial and RS485.

While developing, though, my workstation is a Mac OS - if that may change anything.

---

How do I know it's a FTDI device?

$ system_profiler SPUSBDataType
USB:

    USB 3.0 Bus:

      Host Controller Driver: AppleUSBXHCIWPT
      PCI Device ID: 0x9cb1
      PCI Revision ID: 0x0003
      PCI Vendor ID: 0x8086

        FT230X Basic UART:

          Product ID: 0x6015
          Vendor ID: 0x0403  (Future Technology Devices International Limited)
          Version: 10.00
          Serial Number: DN05Q09Q
          Speed: Up to 12 Mb/s
          Manufacturer: FTDI
          Location ID: 0x14100000 / 8
          Current Available (mA): 500
          Current Required (mA): 90
          Extra Operating Current (mA): 0

---

Using node-serialport library, I have written the following code:

const SerialPort = require("serialport");

const port = new SerialPort("/dev/tty.usbserial-DN05Q09Q", {
  autoOpen: false,
  baudRate: 4800
});

port.on("open", () => {
  console.log("Opened", port.path);
});

port.open((error) => {
  if (error) {
    return void console.error(`Error opening port`, port.path, error);
  }

  port.write(/* Buffer with a length of 23 bytes (proprietary structure) */, (error) => {
    if (error) {
      return void console.error(
        "Error while writing...",
        port.path,
        error
      );
    }

    console.log(
      "Written successfully.",
      port.path
    );
  });
});

$ node script.js
Opened /dev/tty.usbserial-DN05Q09Q
Written successfully. /dev/tty.usbserial-DN05Q09Q

But sadly, while the logs have no errors and I see the activity LED blinking on my USB 2 RS485 device, it appears that the target device does not receive the message as it does not respond as expected (display isn't updating the LEDs).

---

Though, if I am to add a timeout after the write, like so:

port.write(/* Buffer with a length of 33 bytes (proprietary structure) */, (error) => {
  if (error) {
    return void console.error(
      "Error while writing...",
      port.path,
      error
    );
  }

  console.log(
    "Written successfully.",
    port.path
  );

  console.log("Starting a timeout after write...");
  setTimeout(() => {
    console.log("Timeout after write expired.");
  }, 1000)
});

$ node script.js
Opened /dev/tty.usbserial-DN05Q09Q
Written successfully. /dev/tty.usbserial-DN05Q09Q
Starting a timeout after write...
Timeout after write expired.

It works.

Obviously, the device will respond faster than the specified second.
But given that, if I change the timeout to 10ms, it won't work. If I change it to 50ms, it seems to be working at all times, but I have noticed some rare exceptions¹. 40ms seems to work ~5% of the time.

¹ The exceptions occur when changing the payloads. As if some state has been left in the serial port.

---

I also attempted to SerialPort.drain after the write in the write callback. Also tried to drain in parallel.

Neither did help.

---

Sadly, I do not have the production intended SBC to test with that, as it will run Linux and that may change something.

In my debugging efforts, I have tried to watch -n .1 node script.js to see if it's related to some internal buffers of the device, but it works flawlessly with the 50ms. The 10ms one won't register a single time. 40ms produces the ~5% result.

---

Is the timeout a known thing in serial world and there are known patterns to work around that? Maybe this is a MacOS issue? Maybe the RS485 adapter requires extra treatment?

Really out of clues here...