What does opening the standard PC serial port do, in electrical terms (i.e. what voltages on which pins)?
For example, the ancient VB6 program which controls an apparatus I am tasked with maintaining toggles
.PortOpen to control some TTL. The connection only used 2 pins (bad solders fell apart), so which pins do I solder to?
The only labels / documentation refer to pins 7 and 9, saying 0V and 5V parenthetically, but does
.PortOpen really just put 5V between RI and RTS?.
As a post script, this isn't the weirdest thing about the set up. The TTL I referred to above also connects to an instrument via a BNC to DB9 (!), with only 1 pin used. I guess there was an assumption about a common ground, since the BNC shielding isn't connected to the GND pin? The connection is to the instrument's 'foot pedal' pin, it was a way to remotely trigger the device.
According to this page, the DTR and RTS pins can go high when the port is opened. If they were so configured, they will subsequently go low when the port is closed.
If DTR and RTS are not enabled, opening the port should set both to low (and keep them low).