I think the minimum for simplex communication are TX so we can transmit and the power supply pins +Vcc and GND. That is all we need. When do we need to use all 9 pins of RS-232? I think that the reciever can decipher when data has started coming in and if it knows the baud rate already, it also knows when the latch the incoming bits. Therefore, I don't see the purpose of all the remaining pins on the RS-232 cable besides TX, RX, +Vcc and GND. Do we still need them?
The problem is that I wish to connect a PC to a waveform generator through RS-232 cable. I have the software for this installed on the PC but no RS-232 cable. If I do make my own using RS-232 connectors (which is what I intend) with my own wires soldered to it, how do I know if I just need TX, RX, +Vcc and GND or if I need all the other pins as well?
Soldering all the pins is not such a hard thing to do anyway, but I am just curious.
Best Answer
The RS-232 standard was originally specified to support connections between Data Terminal Equipment (DTE) such as computers, teletypes and video display terminals and Data Communications Equipment (DCE) such as modems and Automatic Calling Units (ACUs).
At the time, DCE did not have any internal intelligence, so dedicated signals were designed into the RS-232 specification to manage specific features that were common to such equipment, such as flow control, on-hook/off-hook status and call progress.
Nowadays, modems have their own microprocessors, so it's actually easier (and cheaper) to ignore the dedicated signals in the RS-232 connector and do everything over the serial data lines, using the ubiquitous "AT" protocol.