That would, in part, depend on the size of the text file. If the text file is not too large, then you can use the Read Lines From File.vi and just feed the string output from that vi directly to the Serial Port Write.vi. If the text file is very large, you may have to read it in chunks and send each piece separately.
You may want to try the VISA VIs, but this does not work with all computer/IO card combinations.
Rob