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I have been trying to establish connection between my PC and uC development kit over serial line. There is a standard serial port on the development kit. In oposite there is only USB ports on my laptop. So I have used USB to serial cable for the connection. The problem is that I am not sure whether it is a so called null-modem cable. Can anybody more experienced tell me how to double check it? Thanks in advance.

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It would be very unusual for it to be a null-modem cable. A USB to serial converter normally allows you to add a serial port that is connected in, and works in the same way as the original COM ports fitted to desktop PCs. You would use a null-modem cable to connect, for instance, two PCs together.

If you would need a null-modem cable to connect your development kit to a desktop, you would also need one to connect to your USB converter.

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I manufactured a serial device for 20 years. I have never seen a USB-SERIAL cable with null modem. Look in your System => Device Manager for Ports COM and LPT. It should be there if the driver is installed. The title or the driver may give more information. Most of these cables use the Prolific PL2303 chip.

If you are having problems, its most very likely a driver issue. The serial port and Windows was 97% of my tech support issues.

Try installing the Prolific Driver, the odds are good it will work.

IF that doesn't work Win 10 does a good job of finding the drivers automatically.

It is most likely NOT a null modem cable. It will be a DTE. Not a DCE. IBM designed the PC to be a terminal to their big iron so made it a data terminal rather than data circuit-terminating equipment. The USB cable's serial port is the same as if the PC had a serial port.

A development board made to work with a PC should be a DCE device using a straight through cable and not requiring a null modem unless the designer is an idiot.

The pinout will look like this: (To PC means to USB)
RS-232 pin-out

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  • \$\begingroup\$ Thank you for your reaction Misunderstood. I have checked the Device Manager but unfortunately there is no information about the used chip. \$\endgroup\$ – Steve Feb 23 '17 at 14:01
  • \$\begingroup\$ See my updated answer \$\endgroup\$ – Misunderstood Feb 23 '17 at 14:08
  • \$\begingroup\$ Misunderstood, do you think that my problem is wrong driver? \$\endgroup\$ – Steve Feb 23 '17 at 14:32
  • \$\begingroup\$ I don't think you have a driver installed. In device manager do you have an Unknown USB device? What happens when you plug in the cable? There should be a notification from Windows. \$\endgroup\$ – Misunderstood Feb 23 '17 at 14:37
  • \$\begingroup\$ There is USB Serial Port (COM4) item under Ports (COM & LPT) in my Device Manager. I have also tested the connection via PuTTy and crossover connector at the end of the usb to serial cable. I have written some characters and immediately I have received them back into the terminal window. So I suppose that the driver is alright. I have doubts concerning with the cable. I think that my cable isn't the null-modem cable. \$\endgroup\$ – Steve Feb 23 '17 at 15:07
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I usually don't bother to figure which way it is, I just connect it and if it doesn't work reverse term 2 and 3 on serial end.

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  • \$\begingroup\$ What about DSR and RTR? And RTS and CTS? \$\endgroup\$ – Misunderstood Feb 23 '17 at 15:49

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