Quotation from the Windows 2000 DDK help:
SERENUM is a Plug and Play upper-level
device filter driver that enumerates the following device types:
Plug and Play serial devices that comply with Plug and Play External COM Device
Specification, Version 1.00, February 28, 1995
Pointer devices that comply with legacy mouse detection in
Windows NT® 4.0 and earlier versions
SERENUM is used with Serial, the Windows 2000 system function driver
for COM ports. The combined operation of Serial and SERENUM acts as a Plug
and Play bus driver for a RS-232 port.
The example shows how to use IrpTrace for:
Investigating algorithm used by SERENUM driver to detect device connected to COM port
Determining Plug and Play information returning by Microsoft Serial Mouse device
- A computer running Windows 2000 or XP with one or more COM ports
- Microsoft Serial mouse attached to COM port (COM1)
- Start IrpTrace.
- Click button on the
toolbar (or select Messages|Hook Setup command from the menu).
- Switch to tab PnP. Expand "Ports COM & LPT" item and
click on check box at the left of "Communications port (COM1)" item.
- Click OK to close dialog.
- Ensure that hooks are active (Messages|Activate Hooks button
is pressed) and file logging is enabled (Messages|File Logging button is pressed)
- Run Device Manager applet. Right click on "Communications
port (COM1)" item and select "Scan for hardware changes" command
- Switch to IrpTrace
- Mouse click on last item in the tree. Invoke Edit|Find command.
- Type "Query Device Relations (Bus)", set direction to "Up"
and press Find button. Close Find dialog.
- Expand selected item.
Comparison time, process/tread ID of Create, Close and Set Power/Device Control/Read
requests () shows that Set Power/Device Control/Read requests were sent after
Create and before Close requests. It means that at receiving Query Device
Relations request SERENUM
- Opens lower device
- Sends a sequence of Device Control and Read requests to detect device
attached to COM port
- Closes lower device
Figure 1: Open request sent by SERENUM driver
Figure 2: Close request sent by SERENUM driver
Figure 3: Internal device control request
Sequence of Device Control/Read requests can be separated on 5 groups:
- Configure port
- Serial Internal Basic Settings: disable FIFO, switch of using handshake
flow control and timeouts
- Set DTR/Clear RTS: prevent response from COM device
- Set baud rate: 1200
- Set Line control: stop bits - 0, data bits - 7, parity - none
- Ask COM device to send PnP ID: Clear and then set DTR and RTS signals
- Read ID
- Restore port configuration
IrpTrace shows that SERENUM sent five Read requests. Four of them were completed
successfully (see figure 4) and one with status STATUS_TIMEOUT. So PnP ID of
Microsoft Serial Mouse is:
4D 43 3F 00 | MC?
Figure 4: First byte of PnP ID
Click here for additional information.