A port monitor is responsible for the communication channel between the spooler and the printer device. It supports one or more types of communication channels. For example, the Windows NT default port monitor, localmon, supports parallel, serial, and file ports.
To be able to print to a physical printer, Windows NT requires that the printer be connected to one or more ports that are controlled by port monitors. A port represents the destination for the data being sent to the printer.
Windows NT supplies five port monitors:
| Port Monitor (DLL) | Supported Communication Channels |
| Local (localmon.dll) | Parallel and serial ports; files. |
| Macintosh (sfmmon.dll) | Network adapter card, via AppleTalk protocol, by which an AppleTalk print server or print device is accessible. |
| Digital (decpsmon.dll) | Network adapter card, via DECNet or TCP/IP, by which a DEC print device is accessible. |
| HP (hpmon.dll) | Network adapter card, via DLC, by which an HP JetDirect DLC print device is accessible. |
| LPR (lprmon.dll) | Network adapter card, via TCP/IP, by which an LPD print server or print device is accessible. |
The Windows NT Resource Kit contains a high level overview of the supplied monitors. Vendors need to develop a port monitor if their printing device connects to a different type of I/O port, such as a SCSI or Ethernet LAN port.