I have an IO Pi Plus which was working perfectly well, but failed suddenly. It no longer appears to decode the I2C address:

$ sudo i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 
10: 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 
20: 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 
30: 30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f 
40: 40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f 
50: 50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f 
60: 60 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 
70: 70 71 72 73 74 75 76 77                         

Address is currently set as 22/23.

Is there any chance of repair (what should I try replacing?) or should I consign the card to the junk pile?

Hello Peter

It looks like something is holding the I2C bus low which is why i2cdetect is showing devices on all addresses. Have you tried running i2cdetect without the IO Pi Plus or any other board connected to the GPIO header to rule out a problem with the Raspberry Pi?

If it is a problem with the IO Pi Plus you could try reflowing the solder joints on the connectors and components by holding a clean soldering iron on each joint for a couple of seconds. Hopefully, it is a dry joint or a small solder bridge between the connector pins that is causing the issue.

Hi Andrew, thanks for your response - swift and helpful, as always!

I am convinced that it is the IO Pi plus which has failed- with the IO Pi Plus removed, i2cdetect reports an empty grid. Installing another card reports the expected addresses. The problem occurs on more than one RPi.

Anyway, I will look into reflowing the joints, perhaps the soldering of the 0v pins on the GPIO connector is suspect - heat soak on the ground plane makes the soldering difficult. However, the board was working for several months and it is equally possible that a component has failed. The 23017 drives opto-isolated triacs with mains voltage, but a fault on that driver board (built on vero board!) could feed back to the IOPi. I really need to find some way of producing proper PCBs (with SMDs and with better separation of the LT/HT lines) here in Philippines!

Update:

I cannot find any deficiencies in the soldering, but I do get some strange resistance readings on one of the 23017 chips. Most of the external lines measure around 8Mohm to 0V, but one measures o/c and another measures around 40ohms. Someday I will lift that chip and try again.

Incidently, there seems to be some confusion between your published circuit diagram and the screen printing on the card - R1 against U1 and Bus 2, and R2 against U2 and Bus 1! I suspect that R1/U1/Bus1 and R2/U2/Bus2 is what was intended!

It does sound like one of the IO chips has failed. You could try carefully lifting the I2C pins on the failed chip to disconnect them and see if the other chip appears on the I2C bus. The I2C bus is on pins 12 and 13 on the MCP23017.

Thank you for spotting the mistake with the bus names and the component numbers, I'm not sure how I messed that up. I have updated the schematic so bus 1 is now R2 and U2 and bus 2 is R1 and U1.

Thanks, I'll get my SMD soldering station out and have a go at that sometime. Obtaining and replacing the chip should be easily achieved - RS have a distribution centre in Manila.