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Question: Does the program work with virtual COM ports? USB-COM converters?
Question: Why COM port doesn't open?
Answer: Probably, another program already uses it (the selected COM port). It can be a service application, for example.
Question: What to do?
Answer: Close the application that uses this communicative port (for a DOS application close also a DOS session window). Alternatively, use another communicative port. Probably, a COM port wasn't properly closed.
Question: Is it possible to set variable data transmit rate or transmit 9 data bits?
Answer: No, the Windows operating system doesn't have such the feature.
Question: What socket type to use: DB25 or DB9?
Answer: It does not matter. All modern computers have DB9 only. Alternatively, they do not have DB9 sockets at all.
Question: What cable do I need: straight or crossed (null-modem)?
Answer: It depends on your device. Generally, you should use a null-modem (crossed) cable with the following layout:
Device | Computer
RXD <--> TXD
TXD <--> RXD
GND <--> GND
If a device uses special signals like DTR or RTS, and you don't want to use hardware data transmit control, you need to connect pins 7 and 8 of the DB9 socket on the device side.
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