macrobug

I’ve just bought a ReadyNAS NV+, a NAS box onto which I will do all my backups. It’s great: amongst other things you can log into it using SSH, and use apt-get to install packages, which opens up lots of opportunities such as running my Subversion server on it.

I also wanted to share my old-ish HP DeskJet 940C inkjet from it. Upon connecting it to the USB port on the ReadyNAS, it appeared in Bonjour and I was able to print with it from my Mac - but only using the Gutenprint driver, rather than HP’s own driver. This was not a problem when it was attached to my old Power Mac G4 sitting in the corner… but I’d hoped to retire that. Unfortunately, the Gutenprint driver is slower, doesn’t support different types of paper, nor duplex. It’s good, but it’s not good enough.

I’ve spent a couple of hours researching the problem and have concluded that it’s impossible to solve. Here’s my reasoning. If anyone can find a way round these problems that would be terrific!

How printing works on MacOS X

MacOS X uses a printing system called CUPS. This knows about the printers installed, and manages queues of documents for them. You can control CUPS using the Printers preference page, or by logging into your own Mac on port 631 using a web browser.

The settings for each CUPS printer consist of three things behind the scenes:

1. An entry in /etc/cups/printers.conf which says very little except…
2. A URL to which the printer data should be delivered
3. A PPD (Postscript Printer Description file) in /etc/cups/ppd

The key part is the PPD. This specifies all the capabilities of the printer. It also specifies how to process the data for the printer… and this is done in terms of the “cupsFilter” element. It describes whether the printer can accept a given MIME type directly. If not, it can also specify that the printer can accept that MIME type if the data is fed through a certain programme. CUPS will chain together these “filters” to convert from the original data type to one which the printer can actually accept. For example,a PICT image might be converted to PDF, then to a general bitmap, then to a bitmap format suitable for the printer.

HP Sources

Here are the cupsFilter details for my HP DeskJet 940c…

So, for Gutenprint, a general CUPS raster (which CUPS intrinsically knows how to accept) is converted by the program “rastertogutenprint5.1″ to the data format specific for the printer. CUPS then sends that across the network to the ReadyNas, and all is well.

If you’re using the standard HP driver locally, it uses a command “PrintJobMgr” to send the data to the actual printer. This takes a PDF and does something to convert it to HP’s format.

If you’re using an HP printer connected to a remote Mac, things are very different - it just sends the PDF, PICT or any image format directly to the other Mac - which will then send it through CUPS again, eventually resulting in a PDF form of the data making its way into PrintJobMgr on that distant Mac.

So what’s this mysterious PrintJobMgr? Unfortunately that’s where it all gets a bit depressing. Back in the early days of MacOS X (before 10.2) it didn’t use CUPS - instead it used a “Printer Manager” framework, codenamed Tioga. It appears that PrintJobMgr is a call from CUPS into that older system. That means, the process of converting from PDF (etc.) to the HP-specific data stream is not embedded in a nice executable which we can call from a cupsFilter before sending the data to the ReadyNAS. There’s simply no way to get the data out of the HP driver so that we can send it across the network.

So the HP driver is out - we’d have to run it on the ReadyNAS which means we’d have to run OS X on the ReadyNAS.

What about using “gutenprint.5.1″ to convert to the HP data format, but using the rest of the HP PPD file to configure the available options? I haven’t tried this, but I think it’s a non-starter. According to the Gutenprint user’s manual, the rest of the PPD is very specific to the Gutenprint driver. It specifically says that rastertogutenprint5.1 will fail if you use a PPD other than the one which comes with Gutenprint.

Unfortunately, it seems that to use a DeskJet attached to the ReadyNAS we have no choice but to use Gutenprint.

Update: it’s just occurred to me that the Airport Express is supposed to support printer sharing. I can’t believe it’s running the HP printer driver internally. So perhaps there’s a way… more on that later!

This entry was posted on Saturday, May 17th, 2008 at 2:27 pm and is filed under Techie. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.