powering circuit using serial port

[qtommer]

i would like to power a rs232-TTL converter circuit using the serial port so i dont have to have an external power supply to do the job. I read somewhere that the DTS, RTS and TD pins are able to do this. As I am already using the TD pin to send out data, is there any other way that i can use the DTS or RTS pin to power my circuit?

I understand that i need a 5V voltage regulator but would like to know whether there are any other components that are needed for the job.

Besides that, I am using VB to send out the signals to the serial port. Does that mean i have to send out a series of Logic 0's (positive voltage in rs232) in an infinite loop in my coding in order to get a constant high output voltage?

help would be extremely appreciated=)

 

[creakndale]

Check out this partial schematic for a DataQ DI-194RS data logger.
http://www.ultimaserial.com/misc/di-194power.gif

It uses the PC's 9 pin serial port to power the "D4" 5V regulator. I worked with a DI-194RS and verified that obtaining power from the serial port works.

To obtain voltage, they connect the following together:
Pin 4 DTR
Pin 6 DSR
Pin 7 RTS

Pin 5 is GND

Three things to be aware of:
1) The 3 tied-together pins activate (go high) when the software is started so somehow those pins are controlled by code and I don't know how they do that (I'm not a software person).

2) The UART in some PC's do not drive the RS232 lines with a high enough voltage. The RS232 specification allows an operational voltage down to +/-5V. Through experimentation I found that this regulator circuit requires a minimum input voltage of 6.2V to regulate properly. My laptop would not output a high enough RS232 voltage to operate the circuitry.

3) I don't know how much current you can get out of an RS232 port. It probably depends on the specific UART contained in the PC.

creakndale

 

[duffy]

Tie them together with diodes, any one of them could be driven to 0V by other software or on startup.

 

[ccurtis]

Look at the EDN article at:

**broken link removed**