USB转COM驱动的编写实现有很多中方法,最近在网络上看到一个最常用的方式,即是虚拟一个COM口,在COM初试的时候进行打开一个USB设备.

       当USB串COM口驱动处理Write和Read 等IRQ的时候,其实是去读写USB 设备驱动,读写方式是直接采用ZwReadFile和ZwWriteFile函数.但是真正的实现也不是这么简单,真实的实现是开辟一个线程和一段较大的缓冲区,线程用于适时读取USB设备的数据并保存在缓冲区当中.当上层应用程序向这个虚拟的COM口发送READ IRP(IRP_MJ_READ)的时候,则直接从这段缓冲区中读取数据返回即可.当然还不要忘记处理超时的问题处理.

       因此USB转COM驱动在INF文件加载的时候实际上需要加载两个驱动程序,一个是USB驱动,对应与USB设备的VID和PID,另外一个就是COM虚拟驱动程序,需要对应设置COM口号.如下将对关键代码列出;

• 创建COM口设备对象

status = IoCreateDevice(DriverObject, sizeof(VCP4USB_DEVICE_EXTENSION), &ntDeviceName, FILE_DEVICE_SERIAL_PORT, 0, TRUE, // exclusive &fdo);

• 获取USB设备名称用于打开设备

status = IoGetDeviceInterfaces(pGuid, NULL, 0, &pSymLink); DPrint(DBG_OTHER, ("IoGetDeviceInterface return %d/n", status)); if ((status == STATUS_INVALID_DEVICE_REQUEST) || (*pSymLink == NULL)) return STATUS_UNSUCCESSFUL; pCur = pSymLink; instanceCur = 0; status = STATUS_INVALID_INFO_CLASS; while (*pCur != NULL) { p = pCur; for (size = 0; *p != NULL; size ++) p ++; DPrint(DBG_OTHER, ("No.%d: size=%d/n", instanceCur, size)); DPrint(DBG_OTHER, ("name:%ws/n", pCur)); if (instance == instanceCur) { if (RtlCompareMemory(pCur, pPrefix, prefixLength) == prefixLength) { DPrint(DBG_OTHER, ("Find OK/n")); devName->MaximumLength = size * sizeof(WCHAR); devName->Buffer = (PWSTR)ExAllocatePool(NonPagedPool, (size + 1) * sizeof(WCHAR)); if (devName->Buffer == NULL) { DPrint(DBG_OTHER, ("Allocate devName error./n")); status = STATUS_INSUFFICIENT_RESOURCES; break; } RtlCopyMemory(devName->Buffer, pCur, (size+1) * sizeof(WCHAR)); RtlInitUnicodeString(devName, devName->Buffer); status = STATUS_SUCCESS; break; // find ok and break } } pCur += size + 1; // skip last NULL instanceCur ++; if (instanceCur >= 2) // for debug break; } ExFreePool(pSymLink); return status;

•     打开USB设备端口 

curStatus = GetUsbDeviceName(&usbDeviceName, &GUID_CLASS_XXXX_BULK, &XXXX_SYMLINK_CMPSTR, XXXX_SYMLINK_STRLEN, 0); if (!hRead) rdStatus = createFile(&hRead, &usbDeviceName, L"//PIPE0", 6, GENERIC_READ); if (!hWrite) wtStatus = createFile(&hWrite, &usbDeviceName, L"//PIPE1", 6, GENERIC_WRITE);

    其他的驱动程序处理都是常规处理,在这里就不多做介绍!