I would like to use the following product as an input module(know whether a signal (12~24V DC) is present or not via the GPIO pins).
https://denkovi.com/usb-8-opto-isolated-digital-inputs-module

the protocol
https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/SupportingCollateral/93066A.pdf

I would like to program it in C or C++ (hidapi or external libraries are not wanted).

an excerpt from the terminal

Bus 002 Device 002: ID 04d8:00df  
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass          239 
  bDeviceSubClass         2 
  bDeviceProtocol         1 
  bMaxPacketSize0         8
  idVendor           0x04d8 
  idProduct          0x00df 
  bcdDevice            1.01
  iManufacturer           1 Microchip Technology Inc.
  iProduct                2 MCP2200 USB Serial Port Emulator
  iSerial                 3 0010891880
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength       0x006b
    bNumInterfaces          3
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0x80
      (Bus Powered)
    MaxPower              100mA
    Interface Association:
      bLength                 8
      bDescriptorType        11
      bFirstInterface         0
      bInterfaceCount         2
      bFunctionClass          2 
      bFunctionSubClass       2 
      bFunctionProtocol       1 
      iFunction               0 
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           1
      bInterfaceClass         2 
      bInterfaceSubClass      2 
      bInterfaceProtocol      1 
      iInterface              0 
      CDC Header:
        bcdCDC               1.10
      CDC ACM:
        bmCapabilities       0x06
          sends break
          line coding and serial state
      CDC Union:
        bMasterInterface        0
        bSlaveInterface         1 
      CDC Call Management:
        bmCapabilities       0x00
        bDataInterface          1
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x82  EP 2 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0008  1x 8 bytes
        bInterval               2
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        1
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass        10 
      bInterfaceSubClass      0 
      bInterfaceProtocol      0 
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x03  EP 3 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0020  1x 32 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x83  EP 3 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0040  1x 64 bytes
        bInterval               0
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        2
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass         3 
      bInterfaceSubClass      0 
      bInterfaceProtocol      0 
      iInterface              0 
        HID Device Descriptor:
          bLength                 9
          bDescriptorType        33
          bcdHID               1.11
          bCountryCode            0 Not supported
          bNumDescriptors         1
          bDescriptorType        34 Report
          wDescriptorLength      29
         Report Descriptors: 
           ** UNAVAILABLE **
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0010  1x 16 bytes
        bInterval               1
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x01  EP 1 OUT
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0010  1x 16 bytes
        bInterval               1
can't get device qualifier: Resource temporarily unavailable
can't get debug descriptor: Resource temporarily unavailable
Device Status:     0x0000
  (Bus Powered)

I have tried the simple example, which unfortunately returns incorrect values.
the correct values should be:
idVendor 0x04d8
idProduct 0x00df

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <linux/hiddev.h>
#include <linux/input.h>
#include <dirent.h>
#include <unistd.h>

int main(void){
    int fd=-1;
    fd= open("/dev/ttyACM0", O_RDWR);
    if(fd<0){
        fprintf(stderr,"Can't open device: %sn",temppath);
        return -1;
    }
    else
    {
        printf("DEVICE openedn");
    }

    struct hiddev_devinfo dinfo;
    ioctl(fd, HIDIOCGDEVINFO, &dinfo);
    close(fd);
    printf("VendorID           0x%Xn", dinfo.vendor);  // it returns 0x400
    printf("ProductID          0x%Xn", dinfo.product); // it returns 0x0
}