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I have a StarTech USB-to-RS232C adapter, and I have installed the software properly for it provided by the manufacturer. Device Manager recognizes the device and says it is working properly.

But when I attempt to actually utilize this adapter, I don't get a successful connection between the computer and the RS232 port.

I can't tell why it's not communicating properly. How can I determine whether or not this adapter is functioning as it should?

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So when you try and use a program such as PuTTY what error do you actually get? What COM port does the device show up as? – Mokubai Dec 15 '11 at 14:01
are you using the correct baud rate? bits? error correction, flow control? What are you connecting to? – ppumkin Dec 15 '11 at 16:56
up vote 4 down vote accepted

Two devices I use to test in this case are:

  • an RS232 tester. This is a small block with a bunch of two color LEDS which indicate the state of all the signals. It makes it easy to visually check the state of the connection. Data is visible at rates up to 9600 bps.
  • an RS232 loopback connector. This loops back all the signals to their complement. This allows testing the device in isolation. Looping pins 2 and 3 with a paperclip works well in a pinch.

Failure to have proper DSR/DTR and/or RST/CTS signalling can prevent some devices from communicating. If the device works only when one or both these pairs are looped back, then that is your problem. Normally, this requirement can be controlled locally by your communication software.

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I don't know about your specific adapter but mine has rx/tx lights. I know whether I've a fault with the cable/computer if the lights don't blink when i mash the keyboard in putty.

Are you sure you're pointed at the right com port? - In Device Manager, choose 'Ports (COM & LTP)' and find something like 'USB Serial Port (COM8)'. That bit in brackets is what com port you should be connecting to using your application of choice? (Mine is putty!)

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The best way to test the USB-to-RS232C adapter cable would be to use an loopback adapter.

Using a terminal and disabled echo you should be able to see everything you type.

A simple loopback adapter can be created by simply connecting the rx and tx pin of the RS232 plug.

For details about which pins to connect see here:

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How can I determine whether or not this adapter cable is functioning as it should?

Plug it into another computer.

If it works in the other computer, then your computer has a problem.

If it doesn't work in the other computer, you know that it's the cable with a problem.

EDIT: To determine if the hardware at the end of the cable is at fault, you'd need to use a 2nd adapter cable (then repeat steps above to rule out the 2nd cable also being faulty).

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Good point, but that only rules out the PC. That doesn't rule out the serial end of the hardware. You're right, that would mean the computer isn't the problem but it could still be the adapter or the hardware on the end of the serial connector. – Thomas Stringer Dec 15 '11 at 1:01
Edited my answer. – wizlog Dec 15 '11 at 1:05
I like your answer, and I agree with your approach. I guess I was just looking for any other ways to be able to test adapters (even if in general). – Thomas Stringer Dec 15 '11 at 1:08
If that is the case, consider up-voteing my anser. – wizlog Dec 15 '11 at 1:09
Considered and done. – Thomas Stringer Dec 15 '11 at 1:09

You may want to try the same cable with a different device. What device are you hooking up to the RS232 cable? Try a different device, preferably a completely different kind of device. For instance, if you are trying to connect a mouse, try connecting something like a modem, old Palm Pilot, or whatever else you have on hand. I've heard USB to RS232 adapters don't work at all with certain kinds of devices. Also, are you sure you have the correct drivers for the device you are plugging into the RS232 cable. It doesn't work like USB, you need additional drivers to talk to the device at the other end.

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I don't get a successful connection between the computer and the RS232 port.

Actually you have not provided evidence of this assumption. More likely you are assuming that you have not established a good serial connection to the remote device and its RS232 port. You have not mentioned what RS232 device you are trying to connect. A serial port really doesn't do much without another serial port on the remote end.

How can I determine whether or not this adapter cable is functioning as it should?

If you have a voltmeter (e.g DMM), then you could check the TxD (on pin 3 of the 9-pin connector) is "marking" at about -5 to -15 volts. Ground is on pin 5.

The simplest method would involve first having a working RS232 connection, with a serial COM port on the PC, a "good" serial cable, and a responsive remote device. Substituting the PC's serial port with the USB adapter should preserve functionality, but on another COM port.

Establishing a working RS232 connection is not a plug-n-play operation. You need to obtain the serial port parameters of the remote device, and setup the PC side for the exact same baud rate, number of bits for character length, number of stop bits, type of parity, and method of flow control. Get any one of these wrong/mismatched, and you may get absolutely no data transfers or maybe garbage data.

You also need a proper serial cable. There is no one serial cable that will work in all circumstances. The PC RS232 port will behave as a DTE, data terminal equipment. Typically remote serial devices act as DCE, but that is not guaranteed. Obtaining the serial port pinout for the remote device is always advisable when you cannot make the connection work.

Edit: I once purchased five similar USB-to-RS232 adapters (actually they're converters not adapter cables), and one of them turned out to be a dud out of the box. I don't remember the failure symptoms. So it's possible that you may have a dud also, but you've provided absolutely zero diagnostic information or how you're trying to use or configure this adapter.

I've used USB-RS232 adapters with Prolific or FTDI converter chips. No problems with ten FTDI adapters. One Prolific adapter was DOA. The 5 adapters that use Prolific chips have a RS232 signal ground that floats => Chassis ground of the PC has to be tied to chassis ground of the remote device for a reliable link. The StarTech you have probably uses the Prolific converter IC.

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