For clarification, RTS is an abbreviation for "Ready to Send" and CTS means "Clear To Send".
One of the wonders, and nightmares, of RS-232 is that is was originally designed for communications between mechanical devices such as teletypes (the fact that I once actually used one of those things classifies me as a geezer). Teletypes didn't have input buffers, so a handshake had to be set up between sender and receiver in which the sender would "ask" if the receiver was ready to receive data, and the receiver would acknowledge when it was ready. Two pins on the original 25-pin RS-232 interface were set aside for this handshake.
Modern PCs do not even bother with this trivia any more, in these days of megabit and even gigabit buffers. Under DOS, a programmer could call an interrupt that would set the CTS pin high, thus signaling "I'm a DTE device and I'm ready to receive data". But under Windows, direct access to hardware is no longer possible, since multiple processes nowadays have concurrent access to hardware resources.
The ALGE time base devices I've examined closely still adhere to this handshake, for better or for worse. The TDC will not send until it has acknowledgement there's something out there listening. The Magic Fred-Cable above simply fools the ALGE device by generating a continuous, fake "Clear to Send" signal. Once fooled, the timing device just dumps its data in real time, no matter what.
Originally posted 29 Mar 2002 by James Broder
james@skunkware.tv
