Data is transmitted to the TCA6424A by sending the device address and setting the least-significant bit (LSB) to a logic 0 (see GUID-5351ECA3-D42E-4AE5-AA6D-D066C8CFB2D0.html#SCPS133IMG2619 for device address). The command byte is sent after the address and determines which register receives the data that follows the command byte. There is no limitation on the number of data bytes sent in one write transmission.

The twelve registers within the TCA6424A are grouped into four different sets. The four sets of registers are input ports, output ports, polarity inversion ports and configuration ports. After sending data to one register, the next data byte is sent to the next register in the group of 3 registers (see #SCPS133IMG5008 and #SCPS133IMG3138). For example, if the first byte is send to Output Port 2 (register 6), the next byte is stored in Output Port 0 (register 4).

There is no limitation on the number of data bytes sent in one write transmission. In this way, each 8-bit register may be updated independently of the other registers.