A port monitor controls the I/O port to which the physical printer is connected. For more information about the role of port monitors in the Windows 95 print spooler subsystem, see Typical Printing Process Scenario Using a Raw Spool File.
The local print monitor executable supplied by Microsoft controls parallel and serial I/O ports that may have a printer connected to them. A printer vendor needs to develop a port monitor if their printer connects to a different type of I/O port, such as a SCSI port or an Ethernet port on a network card in the local computer.
If your printer is connected to a parallel (or serial) port and you want to add data, such as printer control information, to the print stream going to the printer then develop a language monitor to do this and use the Microsoft-provided default port monitor instead of developing a port monitor of your own. When a language monitor is associated with a printer driver at printer installation time, all print data that flows from the printer driver to the printer goes through the language monitor before it goes through the port monitor and out to the printer. For information on how to use INF files to associate your custom language monitor with a printer driver at printer installation time, see Printer INF File Extensions.
Sample source code for a port monitor is provided in the Windows 95 DDK. After the Windows 95 DDK is installed, the sample port monitor source code is in the directory \%DDKROOT%\PRINTER\SAMPLES\SPOOLER\LOCALMON.