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Author Topic: PCF8574P or PCA9555 - Creating additional DO  (Read 1394 times)
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Hey guys.  I'm trying to create additional DO for my arduino setup to control some AC relays. I have come up with two options for the circuit.  This is a remote relay board that is less than 10 feet away from main board.  Connected over RJ45 cable & port.  Relays will be driven with ULN2803 (acts as NPN) and a separate 24v power supply.  At this point I'm leaning toward the PCA9555 since its double the outputs in one chip.  Can anyone confirm that the circuit I have setup will work?  

Also, what do I have to do to tie the ground between the 24v supply and the 5v supply?


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Yes they look good to me.
Just connect the grounds of the two supplies together.
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Yes they look good to me.
Just connect the grounds of the two supplies together.

So does it matter which one I go with?  Is there any preference to one over the other?
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So does it matter which one I go with?
Well it will change the software you have to write to manipulate the output pins but both will work equally as well.
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So does it matter which one I go with?
Well it will change the software you have to write to manipulate the output pins but both will work equally as well.

Is there a substantial difference in ease of use / capability of one programming over the other?  I imagine they are similar since they are both on the same bus, but have not have the time to research it.
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Is there a substantial difference
No but they are different.

The PCA9555 is a more modern chip with more registers and improvements. For example it has a polarity reverse register where you can decide on a bit by bit basis if writing a logic one to the bit produces a high or a low. Quite handy for when you are mixing sourcing and sinking devices, so you can get a consistent a 1 turns it on. You also have 8 more bits, it will run at 400KHz if you hack the library, but it is probably more expensive.

The PCF8574, has been around a long time, it was one of the first I2C chips. It has less in it and therefore it is simpler simply because there are fewer options. It will only run at 100KHz.
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