PCF8574 VS SoftWire

Name: "Adafruit PCF8574" Author: Adafruit Sentence: Arduino library for the PCF8574 I2C GPIO expander Adafruit shop Website: https://github.com/adafruit/Adafruit_PCF8574 Name: "EasyPCF8574" Author: Djames Suhanko Sentence: Generic library for PCF8574 easy to use Website: https://github.com/DjamesSuhanko/EasyPCF8574 Name: "IoAbstraction" Author: davetcc Sentence: Treat PCF8574, MCP23017 and Shift registers like pins, matrix keypad, touch screen handler, button press and rotary encoder management (switches) on any supported IO (including DfRobot & Joysticks) with event handling, interchangable AVR/I2C(AT24) EEPROMs. Website: https://github.com/davetcc/IoAbstraction Name: "PCF8574" Author: Rob Tillaart Sentence: Arduino library for PCF8574 - 8 channel I2C IO expander Website: https://github.com/RobTillaart/PCF8574.git Name: "PCF8574 library" Author: Renzo Mischianti Sentence: Most starred PCF8574 library for Arduino (standard and SAMD), Raspberry Pi Pico and rp2040 boards, ESP8266, smt32 and esp32 Website: https://www.mischianti.org/category/my-libraries/pcf8574/ Name: "PU2CLR PCF8574" Author: Ricardo Lima Caratti, [email protected] Sentence: Control the PCF8574 8-Bit I/O Expander with Serial Interface with your Arduino Website: https://github.com/pu2clr/PCF8574 Name: "jm_PCF8574" Author: Jean-Marc Paratte Sentence: PCF8574 Arduino Library. Website: https://github.com/jmparatte/jm_PCF8574 Name: "pcf8574" Author: MSZ98 Sentence: Arduino library for PCF8574 io expander module Website: https://github.com/MSZ98/PCF8574

There are plenty of libraries to use it as a generic I2C i/o device and mix n match input and output, e.g. https://github.com/RobTillaart/PCF8574 is well documented.

I glanced thru the library and see if the rx buffer size is set too low, it will return 0 from your call, looking like it got nothing back. I think you may need to call setRxBuffer() and setTxBuffer. See here