re laplink serial parallel cables serial cables there are only three output signals td dtr and rts there are five inputs signals rd dsr dcd cts and ri there are many differnt null modem requiremts as dictated by the software the ibm bios requires asserts dtr and rts and then waits for both dsr and cts before sending when a bios recieve call is made it asserts dtr and waits for dsr to become true times out after a while and returns with an error if dsr never becomes true it requires cts to be true for sending and recieving most communications packages bypass this and replace it with their own protocol the key is that each comm package could very well have different requirements my favorite cable works in many cases short rts to cts at each end but also run rts through to dcd at the opposite end td runs through to rd and dtr runs through to dsr from both dte s and of course sg goes through to sg i have never had trouble with this null modem even though i have used it with a comm package that was expecting rts to go to cts instead of dcd the advantage of this cable is that it also works with the ibm bios gordon lang
