Plotter waiting for data

Hi,
I have an old pen plotter BBC-SE293 which I want to use. It has a
serial port RS-232C.
Originally it was made to print CAD-drawings from program Mecanic
(Standard-GL language). Because It's slight different than HP-GL, I
will modify the files and send to the plotter via Serial Port.

I made as following:

MODE COM1:9600,N,8,1,P --- same conf. as plotter

COPY C:\ex.plt COM1 /b
or ECHO F1; COM1 --- take pen1

The plotter always displays "Waiting for data".

I tried many different cable configurations founded on the web,
without results.

What have I to do?