Each printer has its own way of detecting printer faults and transmitting fault signals to the LP print service. The LP print service only checks for hang-ups (loss of carrier) and excessive delays in printing.
Some printers provide good fault coverage and can send a message that describes the reason for a fault. Other printers indicate a fault by using signals other than the signals for loss of carrier signal or shut off of data flow. A filter is required to interpret this additional printer fault information.
A filter can also put a print request on hold, wait for a printer fault to clear, and then resume printing. With this capability, the print request that was interrupted does not need to be reprinted in its entirety. Only a filter that knows the control sequences used by a printer can determine where to break a file into pages. Consequently, only such a filter can find the place in the file where printing should start after a fault is cleared.
When a print filter generates messages, those messages are handled by the LP print service. If alerts are enabled, the LP print service sends alerts to the system administrator. For further information, see Setting Up Printer Fault Alerts by Using LP Print Commands.