Figure 2 shows the data flow of a typical command job. Command print jobs typically use a single command filter program defined in the PPD file to generate the appropriate printer commands and handle any responses from the printer. The scheduler also supports a special "command" file format for sending maintenance commands and status queries to a printer or printer driver. CUPS includes backend programs for common direct-connect interfaces and network protocols, and you can provide your own backend to support custom interfaces and protocols. The backend handles communications with the printer, sending print data from the last filter to the printer and relaying back-channel data from the printer to the upstream filters. CUPS includes port monitors for BCP and TBCP, and you can supply your own port monitors as needed.
For example, many PostScript printers support the Binary Communications Protocol (BCP) and Tagged Binary Communications Protocol (TBCP) to allow applications to print 8-bit ("binary") PostScript jobs. The optional port monitor handles interface-specific protocol or encoding issues. CUPS includes a generic PostScript filter that handles all PPD-defined commands. Typically this is used to support advanced job management or finishing functions on the printer. The optional PostScript filter can be provided to add printer-specific commands to the PostScript output that cannot be represented in the PPD file or to reorganize the output for special printer features.
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