PC serial port to VFD speed control ??

Hi,

I'm looking to control the spindle speed of my mill conversion
(running TurboCNC under DOS) by talking to the VFD via the serial
port. (Turbo CNC has a placeholder for customising the spindle speed
command)

Most VFD's seem to be 'RS485' communications and not the 'RS232' your
average PC is equipped with.

Is there an easy / cheap way to convert from one to the other bi-
directionally ?

Andrew Mawson
Bromley, Kent, UK