OK, perhaps we should start again from scratch.
Please explain exactly what you mean by "reed relay inputs".

Hallo Andy,

this thread is a real challenge, isn't?

After re-reading all the posts here, I think we have the following scenario:

Kashyap has reed relay contacts as driver, means reed relay contacts are in external equipment, and Kashyap wants to detect with his 89C52 board whether the contacts are closed or opened.

Well, for this purpose he does not need any opto isolation, because those reed relay contacts already provide galvanic isolation, as you stated over and over again.

But now Kashyap fears, that sensors, contacts or what so ever he wants to connect to his 89C52 board in future, will not necessarily provide galvanic isolation any longer. Also, maybe that these pressure contacts, he talked about, do not contain relay contacts, but some sort of electronic switch, which does not necessarily provide galvanic isolation. Have a look at the following picture, where at the left side an 'Unknown Driver' is shown, containing a NPN transitor as switch:




This NPN transistor can be part of an opto isolator, what would mean that again galvanic isolation does exist. But this NPN can also be a simple transistor, which is connected with it's emitter to ground, maybe even connected to saftey ground (who knows...), and in such a situation, Kashyap would need this opto isolation on his 89C52 board.

In the picture, 'Unknown Driver' contains four outputs: Current source (a bit unprecisely represented by Va and Ra), collector of NPN, emitter of NPN and ground. But also drivers can be found, containing less than these four outputs. Sometimes only the collector and ground is outputted, while having the emitter connected to ground all the time. Many other configurations are possible.

How should the 89C52 board look like to accomplish galvanic isolation in all these cases?
Current loop technique should be used, where either the 'Unknown Driver' is able to deliver the current or the 89C52 board. For this purpose 89C52 board should provide a current source (a bit unprecisely represented by Vb and Rb), an opto LED (LED which is part of a suited opto coupler) and ground.


Let's discuss some cases:

If 'Unkwnown Driver' is a reed relay contact, then a current should flow from Vb through Rb to anode of opto LED of 89C52 board, through its cathode, to the reed realy contacs and back to ground of 89C52 board.
If, on the other hand, 'Unknown Driver' contains this NPN transistor, then a current should flow from Va through Ra to anode of opto LED of 89C52 board, from its cathode back to collector of NPN, through NPN to emitter and finally to ground of 'Unknown Driver'.

In all cases signal wires should be made of twisted pair, perhaps with a cable shield connected to the same ground as it's used for the ground return current.
Current of current loop should be about 20mA.

Kai