Potential issue with serial port???

[wannaBinventor]

I finally got the PL 2303 driver for windows 7 to take. I've got a cheapo USB to serial cable that I ordered from the net. I downloaded tera term (realterm wouldn't install.). I set it for 9600 8N1 and stuck my multimeter com line to the serial port ground and the red probe to the transmit line. If I type something in the terminal window, I just show 5 volts on the TX line when I begin typing and 0 volts when idle. What's weird is that my DMM shows 5 volts hanging for some reason for a period of up to 2 or 3 seconds if I type a few characters -- remember, I'm at 9600 BAUD here so I don't think this should be happening.

I'm wondering how I should go about connecting it to itself in a feedback loop to test it out before I start playing with my PIC on it. Should I just be able to plug it in and connect the TX and RX lines and get some action?

 

[mdanh2002]

Serial port voltage output can be anything between -15V for logic 0 and 15V for logic 1. A digital multimeter may take some time to register the input and stabilize the display, so seeing 5V for a few seconds is normal.

You may want to try a null modem cable to test the serial port. That said, I don't think there's anything wrong with your port.

 

[blueroomelectronics]

Jumper TX & RX

 

[3lectrokid]

using a jumper connect pin 2 to pin 3 in the serial connector and in tera term turn off local echo. type some characters in and if you get characters written back to the screen it works. to make sure disconnect the jumper and when you type characters in nothing should appear on the screen

 

[wannaBinventor]

Hmmm. Thanks for the replies fellas. Looks like I have some issues, because jumping Tx to Rx gives me nothing in the windows nomatter how much I type.

 

[BrownOut]

That's really odd. Double and double-double check your jumper. Are you counting the pins correctly? Are you sure the jumper is making contact with the pins. I've used a sharpened pair of tweezers to make the jumper. You should be seeing something.

 

[mdanh2002]

Are you using a USB to serial adapter? if yes, be careful to install the correct driver for the adapter. Some drivers may appear to install properly but will not work. Once I got exactly the same problem - I could send data using the adapter but could not receive. The problem was resolved when I re-installed the correct driver.

The reason was that some simple drivers are only designed for "normal" devices (e.g. serial mouse, modem) and not meant to be used with home-made circuits connected via the serial port.

 

[blueroomelectronics]

Is the PL2303 driver the correct one for Win7? You mentioned it was difficult to install, why's that?
Are you running 32 or 64bit windows?

 

[3lectrokid]

also double check to make sure there is no handshaking because it will refuse to send characters until the clear to send pin is activated. for ease of use and testing turn off all handshaking and try it on a low baud rate (2400).