Troubleshooting RS232 Signals - Tips & Advice

[Kawakaze]

Hi guys, I am trying to use a USB to serial port adaptor, but I am getting no signal on the pins, I am trying to use the RTS signal to control a switch, but I am getting nothing at all out of it. Any ideas? I've had a scope on it and its just a noisy flatline no matter what its supposed to be.

 

[Bill Simpson]

Move the scope over to the tx data pin and see if it has no noise and is firmly against the rail. (I can never remember whether idle tx is high or low, but I think I remember that the data lines are supposed to be at the opposite rail from the handshake lines) Then see if the tx line wiggles when you blow data at it. Noise on the handshake worries me, those should be solid and clean. I assume you are sharing a good ground between scope and computer and you aren't using a low impedance probe that could leak destructive current levels either direction, true?

 

[Kawakaze]

Hi Bill thanks for the help, I have made a little progress. both the Tx and Rts are high, and the noise I got rid of (loose connection! *facepalm*). The logic seems to be there, but I have no control over it via software.

The project, I should maybe give you some insight. This is a USB to serial port adaptor to control a CCD on a telescope. The current connector is a parallel port, of course no one has a parallel port anymore. I have build a switching circuit, very simple with a transistor.

This didnt work, and on further investigation it seems the port itself is not responding to the software commands.

One more question, correct me if I am wrong, both parallel and usb ports have 5v signals, and a serial port has 12v. how would this work if the serial port is powered from a usb port?

sorry please don't get too technical I am far from a specialist in this kind of stuff. Thanks again.

 

[Ele38]

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One more question, correct me if I am wrong, both parallel and usb ports have 5v signals, and a serial port has 12v. how would this work if the serial port is powered from a usb port?

sorry please don't get too technical I am far from a specialist in this kind of stuff. Thanks again.

Serial port works with signal from +-3V to +-12 (or a little more). The MAX232(which is useful to connect a logic TTL circuit to a RS232 line) chip uses capacitor to make +-12 from 5V, for example. It depends on your adaptor.

 

[Kawakaze]

Solved, you are going to laugh. Seems it was due to compatibility, I tried it on a craptop with WinXP and it works like a charm, the logic is inverted but that shouldn't be too hard to fix. Thanks for the tips guys I certainly picked a few things up.

 

[robotopdx]

Wow it's great to see that this is also a electronics forum! Nice to see you found the answer. Those communications issues can be a nightmare!

 

[rcgldr]

On older PC's I've seen experiements where parallel port pins were used to drive stepper motors via an adapter board to amplify voltage and current. Generally this only worked in a true MSDOS environment where more precise timing was availalble. With a "smart" adapter board, you could use an USB interface that the board splits up into multiple outputs, along with some onboard timers.

 

[madhatter106]

Here's an option for those with PCMCIA ports on their laptops. Quatech makes an adapter that is hardware based instead of software based usb/serial. it's the best way to go when you need that older connection.

I do a fair amount of re-programing and the ECU's are all RS-232 and that's no longer a port on laptops. tried many USB adapters but always fiddling with bit rate speed and drivers not to mention the programing errors from the adapter I had to find another option.

Not 100% sure if this is causing you any grief but I was glad to find a solution to the software usb adapter problem. hope this helps someone else.