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I need some help understanding the pins of I2C 8-bit expander (PCF8574AT) that looks like this,

https://mbrobotics.es/blog/arduino-lcd-1602-i2c/

In the datasheet for this part, there is a pinout diagram for the 16 pin header but nothing is mentioned about the four pins on the top of the module which is SDA, SCL, VSS, and VDD but these already exist in the 16 pin header.

Datasheet for the expander, https://www.nxp.com/docs/en/data-sheet/PCF8574_PCF8574A.pdf

Please help me understand why there are two sets of these pins. Basically, I want to understand how the on-chip IC of the module is wired to the header on its side plus the header on top.

Edit: Added datasheet. Edit: Clarification.

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    \$\begingroup\$ I'm not seeing it in the schematic. Those pins on the 4-pin connector are wired to the chip, some pullups and nowhere else. What else are you seeing? \$\endgroup\$ – Dave Tweed Jan 11 at 18:21
  • \$\begingroup\$ which page in the datasheet are you talking about? .... i do not see any pin headers in the datasheet \$\endgroup\$ – jsotola Jan 11 at 18:41
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enter image description here enter image description here

The 4-pin header J1 is what you connect to your Arduino (or other micro-controller). It has ground, power, and the two I2C lines.

The 16-pin header J2 is what you connect to the HD44780 LCD. 4 pins are not used because the unit is intended to use 4-pbit mode. The header has the same ground and power pins as the 4-pin header, because these must be passed through to the LCD. The I2C lines (scl, sda) are not on this 16-pin header.

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  • \$\begingroup\$ I am using the 4 pin header indicated by the red dotted box which is on top of the module as shown in the picture in the schematic link but there is a "16 pin connection to the LCD" which according to the datasheet also has some I2C pins which is what confuses me. \$\endgroup\$ – Hari Haran Krishnan Jan 12 at 4:27
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    \$\begingroup\$ The "16 pin connection to the LCD" is J2. Which of its pins is you think is I2C? \$\endgroup\$ – Wouter van Ooijen Jan 12 at 9:08
  • \$\begingroup\$ Pin 14 and 15 seem to be SDA and SCL according to datasheet but there is no connection from J1 to any of the I2C header pins (Checked with a multimeter), I think 2 pins of the I2C header are n.c and I noted down the pinout of the header by checking the connection between the chip and the headers. \$\endgroup\$ – Hari Haran Krishnan Jan 13 at 7:57
  • \$\begingroup\$ I think the most probable cause is that you made some mistakes, but there is a chance that your PCB is defective. The situation you describe is surely not 'by design'. \$\endgroup\$ – Wouter van Ooijen Jan 13 at 12:54

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