>We are trying to develop a kit where we need to implement an electrically
>isolated serial port between a kit and
>a standard PC serial port.
>
>We have a circuit using opto-couplers (4N25's) but it is not reliable. It
>seems too
>dependant on the IC parameters so that changing the opto-coupler for
>another of the same type affects whether the circuit works or not. Is has
>to do with the turn-on and turn-off delay times of the opto-coupler.
>
>Does anyone have any suggestions or even a (reliable) circuit to do this?
>There is no power supply available on the PC side except what comes out the
>serial connector.
>
>Any advice gratefully accepted.


It's should just be a matter of designing the circuit for worst case specs.
Optos are notoriously bad for parameter shift over lifetime and also often
have a wide initial parameter spread.
The answers to the following questions would be useful:

- How fast a data rate do you wish to be able to handle?

- What voltage and current is available on the output side?

- What isolation voltage do you require?

- What is the purpose of the isolation eg safety of user, protect PC,
protect output equipment, meet regulations ... ?


regards,



            Russell McMahon

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