Voltage level of DTR in optically isolated RS232 interface

[CMOS]

Optically Isolated RS232

Hi,
I constructed this optically isolated RS232-uC interface circuit but the PC-RXD section (from uC to PC) doesn't seem to work.
**broken link removed**

I checked the voltages on RTS and DTR pins and found both to be at 11.8V. That means no negative supply is available.

What should be voltage level of DTR pin?
Has anyone constructed this circuit?
If anyone has better schematci can you kindly share it?

 

[CMOS]

Re: Optically Isolated RS232

Ok I found the solution. You need to disable DTR pin and enable RTS pin before opening the serial port and that could be done only through programming languages like VB or C.
I was trying simple communication through hyper-terminal in which enabling/disabling these pins is not available. So it was not working.

 

[IanP]

Re: Optically Isolated RS232

First of all I would not relay on supply from RTC and DTR pins...unless you know how to set them, they are useless for you.
So, built yourself +5V to -5V circuit and this should fix the supply problem...

If you still want to try to use power from RS-232 port connect the RS-232 in "null modem" configuration; this will leave you with RTS (7) connected to CTS (8), DTR (4) connected with DSR (6) and CD (1).
Connect diodes (cathode) to RTS-CTS, DSR-DTR-CD and TX and anodes to negative supply and increase C1 value to 10uF.
Now Hyperterminal should work for you...

 

[CMOS]

Re: Optically Isolated RS232

Connect diodes (cathode) to RTS-CTS, DSR-DTR-CD and TX and anodes to negative supply and increase C1 value to 10uF.

Where do I get the negative supply? Do I need to generate it externally?

 

[IanP]

Re: Optically Isolated RS232

Well, if you want to make sure that the negative supply is always available you have to supply it externally and one of good methods will be to use 5V-to-5V isolated converter and use the output as your negative supply (assuming that you have 5V from microcontroller)...

 

[nicleo]

Re: Optically Isolated RS232

Where do I get the negative supply? Do I need to generate it externally?

Since you initially wished to draw 'negative' power supply from PC RS232 port, I presume the 'power' of the negative supply should be low. If that's the case, you may consider TC7660H from Microchip.

Well, if you want to make sure that the negative supply is always available you have to supply it externally and one of good methods will be to use 5V-to-5V isolated converter and use the output as your negative supply (assuming that you have 5V from microcontroller)

You may consider this DCR010505 from Texas Instruments. It's a 5V to 5V 'isolated' dc/dc converter.

Another option:
PT5026
(-)5.2Vout 1A (+)5V-Input
Positive To Negative ISR
Texas Instruments