Duplex printing is when you print on both sides of a page. Some printers which do duplex printing require that you send them special commands to force this mode. This is usually done by the FILTERS. The ifhp filter makes a stab at sending the PJL or PostScript commands to the printer. Many people have reported problems doing duplex printing, so here is a check list.
Make sure you have enough memory for the worst case print job. Usually the printer has to rasterize both pages before it can produce an impression. It may require much more memory than you expect.
Check your printer manual to discover the EXACT form of the enter duplex mode command and make sure that either the command is part of the job (PJL language at the start of the job, postscript header, etc), or that the filter generates the correct form.
Note there is a PostScript Printer Description file (PPD) for most printers that support PostScript, and they even have the PJL and PostScript code for this in the PPD file.
It has been observed that even with what would apparently be sufficient memory, that many duplex jobs print 'oddly', that they are not aligned on the same side in the same way, etc etc. This may not be the fault of the software, but of the support for duplex operation.
Read the ifhp documentation, and create a configuration section in the ifhp.conf file for your printer.
I know this is painful, but until there is a uniform way to get the correct commands extracted from either PPD or some other database then this appears to be the only way to do it.
Patrick Powell