How to determine if plotter's DB9 is intended for joystick/mouseinput?

Hi,

I have this plotter, a Mural 8000 flatbed from the eighties,
brought 2nd hand froma thrift store (no manuals, cables,
etc.) that has both a D25 (female) and a DB9 (male) port.

After some experimenting I found the correct cable pinouts
and currently I have it connected to my Linux box on its (the
plotter's) DB25 (and my PC's 2nd serial port). After some
more experimentation with a total of 14 dipswitches I got it
working, plotting HP/GL files.

It's been said this plotter came with some sort of joystick
or a puck mouse with which it could be controlled. Don't
know what to make of that really -- 'freehand plotting'??

Anyway, the thing is, I really don't know if this was just a
myth or fact. My question is, how can I tell if the DB9 is
basically a DB9 alternative to the DB25 (i.e. for input from
the PC's serial port) or a connector for a joystick/mouse?

I can of course make a DB9 version of the PC-Plotter DB25
cable I have now, and see if that works to get input from the
PC to the plotter. If so, that would prob. rule out an input
port for a joystick/mouse (I'm guessing). But I'm kinda
apprehensive, cause I don't want to run the risk of destroying
my PC's serial port if the DB9 on the plotter is not intended
for this. Clearly I'm totally unexperienced in all this.

I've tried to trace the pins on the DB9 on the plotter to
various components. As far as I can tell, the 'first'
components that the various pins trace to are these:

Plotter DB9 port (male):
1 to pin #3 of an 8-pin resistor network a472ga
2 to pin #2 of an 8-pin resistor network a472ga
3 to pin #5 of an 8-pin resistor network a472ga
4 to pin #4 of an 8-pin resistor network a472ga
5 to 1st switch of dipswitch block no.1 (which has 4 dsw)
6 to pin #6 of an 8-pin resistor network a472ga
7 -
8 to base of transistor MPS6530
9 -

I am not 100% sure if pins 7 and 9 are really unconnected, the
connector's housing is in the way, and using a multimeter and
shining a lamp from underneath and above the pcb weren't
conclusive. All instances of the resistor network mentioned
above is one and the same component.

I might add that I did in the meantime muster the audacity to
hook up an old serial mouse (Genius Netmouse) and some non-
descript DB9 joystick (a Tecnoplus [sic] TP135), and fiddled
with some dipswitches (blindly, for I have no docs) but the
plotter just sat there.

Any hints, tips and what have from you much appreciated! I'm
ready to experiment, and like I said, have multimeter (will
travel I almost added).

Thanks!

bj