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powering circuit using serial port

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qtommer

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