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Post: Hi Tony If there is only 2V at the power input pins then that suggests there could be a problem with either the power supply not supplying enough voltage or the IO Pi is drawing too much current causing the voltage to drop. Can you try measuring the amount of current the IO Pi is drawing by connecting your multimeter in current mode between the 5V wire from the power supply and the 5V pin on the IO Pi power connector? The IO Pi should only draw a few milliamps when none of the IO pins is set as outputs so if it is drawing a lot of current that would suggest that one or both of the IO chips are faulty. You could also try touching the chips to see if they are warm as that would also suggest a fault. Also try checking the continuity between the ground output on your power supply and the ground pins on the Raspberry Pi GPIO connector, or the outside of the Ethernet or USB sockets, to make sure there is a good connection between the grounds on the two power supplies. The fact that the IO chips are showing up on 0x20 and 0x21 I2C addresses suggest that they are working at least to some degree as faulty chips normally wouldn't show up on the I2C bus so it could be a power supply fault that is causing the problems. The MCP2317 IO chips will work from 1.8V to 5.5V so if your power supply is only providing 2V the chips would appear on the I2C bus but they would also only provide 2V on the output pins which may not be enough to drive your Darlington arrays. If you are only using the IO Pi to drive Darlington arrays then it should be safe to use the Raspberry Pi to power the IO Pi instead of an external power supply. Darlington arrays like the ULN2003A should draw less than 2mA from each IO Pi output so it will put a minimal load on the Raspberry Pi's power supply.