I would suggest using a USB-parallel converter. As a result there are less cases to find of success.
![usb parallel adapter driver windows 7 usb parallel adapter driver windows 7](https://i.ytimg.com/vi/K4PqROjbIjY/maxresdefault.jpg)
Further, not many folks these days are using usb-serial connections for printers in Windows. There are just too many variations in serial connections, not to mention the baud rate and other things that need to be set correctly. The Designjet 450c does have a serial connection capability, but you are likely to have much better chances of success with a parallel connection.
![usb parallel adapter driver windows 7 usb parallel adapter driver windows 7](https://tandd.com/img/information/reinstall/7-2.png)
You seem like the person that would have the answer for this. I have tried to set up a port, but ran into a road block there as well because the driver did not have a digital signature. I do see that the device is labelled "Port_#0002 Hub_#0003" and am wondering if I need to set up a port some how. In following that trail, I have not found the way to bind the driver (to which you led me that worked for the network setup) to the device. There is also an alert symbol on the device icon which is there because of the absence of a driver. I have found that simply plugging in the cable yields this message regardless of connection to or powering up of the plotter. It shows up as an "Unspecified Device" under Printers and Devices with the notation "USB 2.0 to COM Device". I purchased a USB to RS232 adaptor hoping to accomplish this, but Windows 7 does not recognize the printer.
![usb parallel adapter driver windows 7 usb parallel adapter driver windows 7](https://assets.aten.com/product/image/UC1284B-USB-Converters-OL-large.jpg)
USB PARALLEL ADAPTER DRIVER WINDOWS 7 HOW TO
Your other post enabled me to print on it over the network by showing me how to get a driver, but I am not having any luck connecting directly to the plotter. We have an HP450C that was formerly the local printer on an XP computer and I am currently trying to run it as a local printer on my new Dell. I found one of your other posts helpful in solving a network printer problem after purchasing a new Dell with Windows 7.