software and +5V exactly — nothing. he has a voltage changing from +5..+15 for "0" and -5..-15 for "1". including catches you there is likely something in the area (±)7..8V
view diagrams online(google: rs232 5v), which will give you a smooth 5v, and then search for softwae decision as to the pin to give the time 1. (however, afaik, you will still get "saw", because, depending on the frequency will still drop to "logincache unclear" (+3v)). in General, no additional schema does well