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Registry Settings for a Plug and Play Serial Device

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This topic describes the registry settings that Serial uses as a function driver for a Plug and Play serial device. Serial also uses these settings as a lower-level device filter driver for a device that requires a 16550 UART-compatible interface.

Serial queries these registry entry values when it adds the device. If a device-specific entry value is not present, Serial uses a Serial service value.

The following registry settings are under the Plug and Play registry key for a device.

PortName (REG_SZ)
Specifies the name of the device. The name of a device is typically COM<n>, where <n> is a COM port number that the installer obtains from the COM port database. However, the device can be set to any non-NULL string. If the device is configured as a COM port, Serial uses the port name to create a symbolic link name for the device. The default value of PortName is an empty string.

Identifier (REG_SZ)
Specifies the name of the device. The support for an Identifier entry value is provided for compatibility with some legacy PCMCIA devices. The use of Identifier is obsolete and should not be used with Microsoft Windows 2000 and later drivers. For a description, see the PortName entry value.

MultiportDevice (REG_DWORD)
Specifies a Boolean flag that indicates whether a serial port is a device on a multiport device. If MultiportDevice is 0x00000000, the serial port is a stand-alone device; otherwise, the serial port is on a multiport device. The default value of MultiportDevice is 0x00000000.

PortIndex (REG_DWORD)
Specifies the index number of a serial port on a multiport device. The Indexed entry value specifies whether a port is bitmapped or indexed. The default value of PortIndex is 0x00000000.

ClockRate (REG_DWORD)
Specifies the UART clock rate. The default value of ClockRate is 1,843,200 Hertz.

Indexed (REG_DWORD)
Specifies a Boolean flag that indicates whether a port on a multiport device is bitmapped or indexed. If Indexed is nonzero, the port is indexed; otherwise, the port is bitmapped. Indexed is used in conjunction with the PortIndex entry value. The default value of Indexed is 0x00000000.

DisablePort (REG_DWORD)
Boolean flag that specifies whether to disable the device. If DisablePort is nonzero, Serial disables the device; otherwise, the device is enabled. Use of the DisablePort entry value is obsolete and should not be used with Windows 2000 and later drivers. Windows 2000 provides a generic manual method through the GUI of Device Manager to enable and disable devices. The default value of DisablePort is 0x00000000. Note that flagging a device as disabled does not mean that the device does not exist. Serial still attempts to detect the presence of a disabled device. If the device is specified as disabled, Serial returns STATUS_NO_SUCH_DEVICE in response to an IRP_MN_START_DEVICE request. After the start request fails, the Plug and Play manager sends a remove request.

ForceFifoEnable (REG_DWORD)
Specifies a Boolean flag that indicates whether to force Serial to use FIFOs. If ForceFifoEnable is nonzero, FIFOs are used, regardless of whether Serial can detect the presence of FIFOs. Otherwise, FIFOs are only used if Serial can detect them. The default value of ForceFifoEnable is the value set for the Serial service. (The default value for the Serial service is 0x00000001.)

RxFIFO (REG_DWORD)
Specifies the number of bytes in the receive FIFO that triggers a serial port interrupt. For valid values, see the constants defined in the Serial.h header file in the Serial driver sample on GitHub. The default value of RxFIFO is the value set for the Serial service. (The default value for the Serial service is eight bytes.)

TxFIFO (REG_DWORD)
Specifies the number of bytes in the transmit FIFO that triggers a serial device interrupt. For valid values, see the constants defined in the Serial.h header file in the Serial driver sample on GitHub. The default value of TxFIFO is the value set for the Serial service. (The default value for the Serial service is fourteen bytes.)

MaskInverted (REG_DWORD)
Specifies a Boolean flag that indicates whether the serial device hardware inverts the contents of the interrupt status register. If MaskInverted is nonzero, the interrupt status register is inverted; otherwise, the interrupt status register is not inverted. The default value of MaskInverted is 0x00000000.

SerialSkipExternalNaming (REG_DWORD)
Specifies a Boolean flag that indicates whether Serial configures the device as a COM port. If SerialSkipExternalNaming is set to 0x00000000, Serial configures the device as a COM port; otherwise, Serial does not configure the device as a COM port. The default value of SerialSkipExternalNaming is 0x00000000. For more information about how Serial configures a device as a COM port, see External Naming of COM Ports.

SerialRelinquishPowerPolicy (REG_DWORD)
Specifies a Boolean flag that indicates whether Serial is the power policy owner for a serial device stack. If SerialRelinquishPowerPolicy is zero, Serial is the power policy owner; otherwise, Serial is not the power policy owner. The default value of SerialRelinquishPowerPolicy is 0x00000000.

Share System Interrupt (REG_DWORD)
Boolean flag that specifies whether to permit the system to share the interrupt that the device uses. If Share System Interrupt is nonzero, the interrupt can be shared; otherwise, the interrupt cannot be shared. The default value of Share System Interrupt is the value set for the PermitShare entry value for the Serial service. (The default service value for PermitShare is 0x00000000.)

SerialIoResourcesIndex (REG_DWORD)
Specifies the index of the partial resource descriptor that Serial uses to determine the I/O addresses of the serial register set for the device. The default value of SerialIoResourceIndex is 0x00000000.