Hi
I've recently been spending a lot of time with a HP 7550 pen plotter. Very much like your unit, perhaps a few more bells and whistles. Complete documentation for just about any HP device or software can be had from
http://www.hpmuseum.net/collection_document.php
I also have Acad 2006, and have used that Printer driver on a Graphtec pen plotter that accepts such connections The Graphtec also runs on HP-GL commands.(It's OLD ;-).
I assume you are trying the serial connections.
The HP 7550a unit I have provides connectivity for Serial connections RS-232, or the HP-IB/GP-IB parallel interface. No "centronics printer port connection" . The serial connection requires a "null modem" configured chord.
The HP-IB cards I have are not on a box that I connect to the plotter yet.
My setup is using Linux and a terminal window to send files to the plotter. That works Very well. I have made use of both pstoedit and hpgl -distiller applications.(
http://pldaniels.com/dxf-hpgl/) to format and clean up the files for straight plotting.
HPGL/2, which is a contemporary vector graphics output has more commands than does the older HP-GL conventions. Some machines, perhaps yours, will "choke" on some of the commands. The 7550a does cough over the unrecognized commands, sending out a "command not recognized" warning. Plotting can resume after a clear using the panel key board. Even then, at times that confusion disconnects the serial terminal connection, and a restart is needed.
I have gotten around nearly all the command headaches by using the hpgl-distiller, but I still like to review the files using a text editor before sending them to the plotter. Page size, scale and formatting is still in developement for me.
At this time, I use Inkscape for generation of most plot files, some from bitmap or jeg image files. First saved as an .eps (encapsulated post script) then to the pstoedit program for conversion to HP-GL. then to the distiller for cleanup. Acad drawings are not something I generate every day, nor do I have ACad on the Linux box.
There are a lot of HP-GL converters for Windows, You can search the net.
Oh, I did spend a bit of time using the USB to serial converter. I have a Targus device that works just fine on my Win XP IBM R-51 thinkpad. The Realterm application is my choice for setting up the serial connection, and transferring files or commands.
Wish you good fortune
Cal
PS a .plt file IS a HPGL format file. Same, same!