Does a board connected to serial port need power?

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Thread Starter

ndtoan

I'm making a board connected to serial port but don't want to make an individual power for it and want to use the voltage avaiable from serial port. Does anybody know how to do it?

Thanks a lot.
 
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Curt Wuollet

Easy, don't put any active electronics on it. :^). For very low power requirements, one can steal power from the signals themselves, using diodes for isolation and caps for storage. I believe in most cases, that is how it's done.

Regards

cww
 
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Robert Scott

The power from the signals on a serial port is very limited - about 9 volts at 2 ma. in some cases. It depends on the computer board. Some serial ports are capable of a little more than that. Use diodes to combine the RTS and DTR, and maybe even the TXD signals to get the maximum power. If you are programming the PC end of things, make sure to set RTS and DTR high. TXD will only go high during a transmission, so it is not as useful.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting
 
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Lynn at Alist

Well, the EIA/RS-232 spec requires a receiver to appear as 3000 or more ohms, so a +5v signal into a 3000 ohm resister means you can only trust having about 1.5mA of power from every RS-232 port. Beyond this the RS-232 chip is permitted to start "current limiting" itself. You'll see the voltage drop off, so that with no load most RS-232 ports are at +/-9 to 12v, but as the current load increases the voltage will drop off until you find it unusable. I did some measurements once and the range
for common equipment seemed to be about 5mA to 15mA when just shorted through an DVM.

But your board will always be product dependant - (just to pick 2 random notebook suppliers) what works on say a Toshiba serial port may not work on an Acer, etc.

How to do? Just buy any of the existing "port-powered" product on the market and see how they combine the + and - voltages. Many use a voltage-inverter since a -V will be more common and powerful on RS-232 ports.

- LynnL
 
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Johan Bengtsson

It have been quite a while sice I read the RS232 spec but I think the lowest voltage is somewhere around 6V and the maximum is somewhere around 23V. For a general device it would need to handle this.

The voltage is usually around 12V, but that is not a RS232 requirement.

The lowest voltage as input signal that still should be detected properly is by the way (if my memory is right) 3V.

/Johan Bengtsson
 
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