Q. I need a 20m RS232 cable from my DR66 to my laptop. Is this possible?

A. The maximum recommended length of the data cable between the PC and the DR unit is 5 meters. 

This is because the RS-232 is an unbalanced connection, and the signal will be subject to loading as well as induced noise into the cable, meaning that a longer version of the supplied cable is not likely to work.

There is a way around this, however. You could convert the RS-232 to a screened,
balanced format (RS-422 / RS-485 etc.) cable for the length of the cable run from the PC,
then convert back to RS-232 before connecting to the DR.
This can be accomplished with the use of proprietary converter boxes. 

Because the only pins in the connection that are used are pins 2, 3 and 5 (Tx, Rx and GND),
there is no means to use any other of the control lines to power the converter boxes, so you would need to find some that are battery powered.  

Once this is done, you have a much greater capability to communicate with the DR over distance, such as 500m etc.
To be as effective as possible, the unbalanced part of the total connection would need to be as short as possible.