A language monitor allows the spooler to configure and monitor the status of a bidirectional printer. A bidirectional printer supports two-way communication between itself and the print spooler running on the computer to which the printer is connected.
Using a language monitor, the spooler can request configuration and status information from the printer, and/or the printer can send unsolicited status information to the spooler whenever certain events occur on the printer. For example, a language monitor could be written for a printer that can detect and communicate events such as paper jams, no paper, low or no toner, and/or the fact that the printer is warming up and not yet ready to print.
Windows NT supplies the following language monitor:
| Language Monitor (DLL) | Printer Language Supported |
| PJL monitor (pjlmon.dll) | Printer Job Language (PJL) |
If a printer manufacturer’s device is bidirectional and uses a language other than PJL, vendors can develop a language monitor for it. A language monitor can also be developed when a vendor wishes to add data, such as printer-specific control information, to the print stream going to a unidirectional printer. Language monitors are completely optional in the Windows NT print architecture.
For the printer to understand requests from the computer and for the computer to understand solicited or unsolicited responses from the printer, a vendor must define a language. For example, the HP LaserJet 4Si is a bidirectional printer device and Printer Job Language (PJL) is the language that implements its bidirectional communication.