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PCF8575: what is reading slave address

Part Number: PCF8575
Other Parts Discussed in Thread: C2000WARE

Hello, I am developing the driver for IO expander PCF8575. The spec says the writing address is 0x40 and the reading address is 0x41. But I scanned the I2C device and got the slave address is 0x20. If 0x20 is the slave address, then which address should be used to read from this device. Thanks for your help!