Question: Why COM port doesn't open?
Answer: Probably, some other program already uses it (COM port). This can be DOS application, for example.
Question: What to do?
Answer: Close application, using this communicative port (for DOS application close also DOS session window). Or use other communicative port. Probably, at start or some program stop was made some fatal error and COM port wasn't properly closed.
Question: Is it possible to set variable data transmit rate or transmit 9 data bits?
Answer: No, Windows operation system doesn't let such liberties.
Question: What cutoff point type to use: DB25 or DB 9?
Answer: It makes no difference, select only in program corresponding COM-port. USUALLY DB25 - COM2, DB9 - COM1
Question: Is cable connection direct or null-modem?
Answer: Everything depends on your device cutoff point. Usually it is necessary to use null-modem cable, in which signals are moved apart in this way:
Device | Computer
RXD <--> TXD
TXD <--> RXD
GND <--> GND
if device uses special signals DTR and others and you don't want to use hardware data transmit control, at device side connect 7 and 8 contacts of DB9 cutoff point or analogous DB25 cutoff point signals.
More hardware tips and articles you can found on our site http://www.aggsoft.com.
Question: Is data receive becomes at once after program start or data receive must be started?
Answer: Everything depends on your device type, usually in device settings is selected to echo view data to COM port. If your device doesn't support this mode, write what initialization string must be sent to read out data, and we add this possibility to the program.