isolated power for parallel port optocouplers

hey guys,

I'm trying to create a totally isolated parallel port drive interface circuit for a particular machine.

I want to use optocouplers for both inputs, and outputs. Outputs are fine, low power optos run directly off the data lines. The problem i run into is the inputs to the PC.

So, i have 2 questions:

1. I notice that several of the input pins are labeled "inverted" and some are regular. Do i have to drive them high, or low or some mixture of each?

2. How can i get power to drive the inputs to the PC high, while remaining totally isolated, without using an external battery? I was thinking using an isolated DC-DC converter 5V in, 5V out, and connected to the machine 5V logic line. Would that give me the isolation i'm looking for? any other way to do it? DC-DC converters are kind of pricey, kind of large, and i'm not swift enough to design my own.

All you need is a small ac transformer (18 VCT gives you 9 VAC off the CT and one leg), 250ma to 1A rating; a small 1A or 1.5A round bridge and a 1000mfd @ 25VDC (or higher) filter cap. Feed a 7805 with a .47 mfd across the output and you have the "floating supply". It's ground return is not connected except to the circuit on the input of the opto side.

Since most noise comes across via the common ground connection getting true isolation requires a separate source of DC. A DC-DC converter only works if it has a transformer and is isolated. It is the most expensive way to go if any current at all is needed.

Another really cheap option is to buy a 5VDC wallplug transformer. It doesn't even need to be exactly 5VDC. You just figure the current limit resistor in series with the opto input emitter based on the DC voltage in the circuit.

tomCAUDLE
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I wanted this thing to be totally self-contained. No power packs, no external cords, etc... The only power I have available inside the machine at the interconnection point is the +5vdc logic supply, and the parallel port can't provide any power of it's own. (Why didn't they just include one 5V pin???) Since the power from inside the machine is already DC, the suggestion above won't work.

I really don't need much current at all. Just enough so that the PC sees a logic high.

Some of the DC-DC converters are isolated. Looks like they chop up the input DC, effectively turning it into AC, and run it across a transformer, and filter it back to DC again

I'm just looking for a completely isolated connection. Input and output. Would i be safe with just a direct connection? Probably, but i'd rather be safe, and account for every possible mode of operation and failure, than have a fried pc.

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Originally Posted by mike944

I wanted this thing to be totally self-contained. No power packs, no external cords, etc... The only power I have available inside the machine at the interconnection point is the +5vdc logic supply, and the parallel port can't provide any power of it's own. (Why didn't they just include one 5V pin???) Since the power from inside the machine is already DC, the suggestion above won't work.

I really don't need much current at all. Just enough so that the PC sees a logic high.

Some of the DC-DC converters are isolated. Looks like they chop up the input DC, effectively turning it into AC, and run it across a transformer, and filter it back to DC again

I'm just looking for a completely isolated connection. Input and output. Would i be safe with just a direct connection? Probably, but i'd rather be safe, and account for every possible mode of operation and failure, than have a fried pc.

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Some of the Analog Devices' isolators from the ADUM1400 series have a DC-DC converter included into the chip, some have input and output going channels.

That's a very interesting device. I may use that instead of my optocouplers. I wish it wasn't surface mount though. Oh well, i guess i can't be too picky

What kind of signals are you wanting for your inputs? If they are mechanical switches and the connections and contacts have no conductivity to anything else, they are already isolated, no need for anything.

Closed switches come out of the control distribution board as +5v, which is tied to the machine supply.

Then if you want to be isolated you will need either opto's or other isolators.