There are several DIY circuit projects that converts data coming from RS232 port to DMX-512 format. Typically those devices consists of a microcontroller that reads some simple commands that modify the different channel data values on the microcontroller memory. And the program on microcontroller periodically sends the data in it's memory buffer to the DMX-512 out.

Here are links to many DMX-512 circuit projects (RS-232, parallel port and USB interfaced):
http://www.epanorama.net/links/lights.html#dmx512