Positive voltage power supply from RS232 help needed

Hello everybody,

I am working in a project for school. I have to design a RS232 RS485/RS422 converster self powered. I have just the Rx, Tx and GND pins.

The main difficult thing is to create a power supply from Tx pin because Tx pin is -12V idle.

At first, I have create a inverting power supply with NE555 like the shematic enclosed. but when we put a charge (supply of the components 232 and 485), the voltage collapses.

Is there an over solution for create a positive power voltage from the TX pin ?

I buy a RS232 RS485/RS422 converster self powered on a french website and it work very well just with Rx, Tx and GND. So it's possible to powered just with the Tx pin.
This is the datasheet of this converter : **broken link removed**

I need your help.

Thanks

And exuse me for my poor english

I assume you manage to generate +5V until you load it?

It is tricky to be certain what the problem is but I think you need to find out the current consumption of the chips. That will determine how large the storage capacitor needs to be - it needs to hold up the power during data transfer.

Also, what frequency are you clocking at? Your C7 looks strange - it doesn't look like a capacitor, but maybe that is just a resolution problem on the image.

You may find you need C8 to be considerably larger.

Also, monitoring the supply under various loads may give a clue to the amount of power your inverter can supply. You might do better with a charge pump chip rather than making your own with a 555. The will probably be a lot more efficient.

Keith.