1. Serial Mice;
2. COM Modems;
3. Other COM devices;

The main difference between them is a way, how these devices support enumeration. On the PC's side the PnP manager tries to execute certain steps at the certain time .Perhaps it sounds not clear, therefore if you want to understand this process better, please see WINDOWS 2000 DDK , "Features of Serial and Serenum","Inside Microsoft WINDOWS 2000" book (David A. Solomon, Mark E. Russinovich) , Microsoft Press, chapter 9 "I/O System", an excellent book, containing the full description of this process (additionally, chapter 4, "Startup and Shutdown").
At first let's speak about the first type (SERIAL Mice, the most primitive type), the full description of all three types it is not goal of this document, the specification itself contains all necessary information.
In order to a serial port based device could be identified as a serial mouse by PnP Manager(with help of I/O manager) the next steps have to be executed (it will be used in PART 3):

1. echo DTR to DSR, always in hardware (everything what is received from DTR has to be sent to DSR, it can be direct connection);
2. while (RTS ! = 0) { just wait, unless RTS = 1};
3. if (RTS = 1), identify itself, sending COM ID (fixed structure);
4. go be a mouse, doing something (in my case the prototype just moves the cursor in a certain path (rectangle), with uniform speed);
5. if (RTS = 0) GOTO step 2;

Our new created device has to report about itself at the speed of 1200 bit/s. The standard strictly defines the data formats, which should be used: nine bit frame: one start bit, 7 data bits (LSB first) and one stop bit. Examples of compatible formats are:

1. nine bit frame: one start bit, 7 data bits, no parity, one stop bit;
2. ten bit frame: one start bit, 7 data bits, no parity, two stop bits;
3. ten bit frame: one start bit, 7 data bits, Mark parity, one stop bit;
4. ten bit frame: one start bit, 8 data bits with MSB set to 1; one stop bit;

Only one momen is not described yet, the data ( COM ID ), which has to be sent to the PC from the external serial port based device structure. This information is exactly one which allows to OS (PnP manager) distinguish among different devices, obtain and show the certain information during enumeration, load necessary drivers and so on.

Table 1, Plug and Play COM Device ID Fields
(Plug and Play External COM Device Specification rev. 1.00, page 10)

The items, having blue color are mandatory attributes. The detail descriptions of these fields you can find on the specification,(3.2 Definitions, pages 11-12).
Now lets try to preliminary fill this structure with our own values, for future usage with software emulator (PART 2):

1. PnP Rev = 1.08. Any value which suits this filed requirements (see specification);
2. EISA ID = SRD (a unique three characters EISA identifier);
3. Product ID = 0003 (32 bit value in 8 character hexadecimal code);
4. Serial number = 01234567;
5. Class Name= MULTIFUNCTION;
6. User name = "Virtual serial port based device";

I used these parameters without any special intentions, for example "SRD" is "Serial poRt Device", Product ID was taken as 0003 just for fun, but anyway, later I used these data in my INF file, therefore now, if you want to use offered by me software, you have to leave all these values as they are, otherwise you should make certain correction of all necessary part.

The last item (User name) will be shown on the test PC in the window ("Found New Hardware"), during installation of the driver of our virtual device, therefore this is a good idea to make this information understandable and meaning something.
In order to finish the first part (the main description), I would like to say a couple words about the way, how OS determines the presence of the new device on the serial port. The OS, (WIN2K in my case) can discover the presence of the new serial port based device during loading(restarting) OS or only if user activates "Search new hardware"("Scan for hardware changes") through Device Manager (these two ways are obvious enough). OS itself does not see changes on serial port; this is different from USB devices management (for example), when OS (not directly, but with help of bus driver and other related software parts (I/O manager, PnP manager)) respond to all changes on USB immediately. For now, this is just a notice, it will be used later. If you are interested in getting more information about enumeration-based drivers loading procedure, please check "Inside Microsoft WINDOWS 2000" book (David A. Solomon, Mark E. Russinovich) , Microsoft Press or similar literature.

PART 2