Why is it that despite listening on TCP 9100, some printers will just print the source code when you send them a PDF or Postscript file with netcat?
Is there some special PJL command to put it into Postscript mode? Are there any networked printers around nowadays that don't support Postscript?
Yes, there’s a protocol behind that port. Just like there’s one behind port 22, 80, 443... you can’t just dump http to 443 and get a useable document in return.
Not on 443, but on 80 you basically would. You'd just have to skip the response headers.
@PJL ENTER LANGUAGE = POSTSCRIPT
Yes.
Not all printers interpret those languages on that port. For example, Zebra printers interpret ZPL there, and I think most Postscript would look like plain text to a ZPL interpreter.
> Are there any networked printers around nowadays that don't support Postscript?
I'd be somewhat surprised if most label printers and receipt printers accept Postscript. I wouldn't be surprised if there are more niche types of networked printers where it might not make sense to support Postscript.