Trying to understand my plotter

Ive been given an A1 oce 5140 plotter, my first ever (for hobby use),
probably dated from the mid '90s and Im having problems understanding it's
coordinate system when using it from the PC. Ive RTFM which doesnt explain
anything decently.

It has 5 possible XY origins, bottom left,bottom right,upper left,upper
right and centre.
It can understand several different protocols including HP-GL and HP-GL/2
and works out which one automatically when the PC communicates with it
across RS232 port.
For each protocol, the XY origin differs. These are configured manually on
the oce

Why are there different origins? even hp-gl and hp-gl2 default values are
set differently to each other

Using the plotter is totally baffling.
First, it uses a roll. Yet there are non printing margins specified in the
manual. Now a Left non printing zone I can understand, but why bother with
right, top and bottom when it accepts paper upto 2ft wide and moves the
sheet up and down as it needs to? (AND it supports long plot for paper upto
2 meters)
When it gives the margin values, which one is "top","bottom" etc when the
coordinate system can be all over the place? On the hardware Y is the
default horizontal axis and X the vertical btw
One of the biggest bores has been actually plotting to the damn thing.
However much I play with the computer settings, I nearly always have to add
an offset to the X and Y to get the plot on the paper. Of course the PC then
removes this from the top and right of its own page. The preview never looks
like the final page. I had to design a very custom sized sheet of paper to
get the full print on the real page.
Swapping between portrait and landscape produce all sorts of interesting
results - the plot goes backwards, upside down, doesnt change, gets cut off
after a couple of inches ad nauseum.

Its all horribly confusing.

If anyone recognizes these problems and can help then please help me.
If anyone knows of where I can get a free hp-gl/hp-gl2 language reference so
I can write some scripts to test it, Id be even more grateful