N-Ch FET with open-drain output
The data that you shift in on SDI will be shifted out on SDO. You can optionally connect multiple of these devices in series:







simulate this circuit &ndash; Schematic created using CircuitLab

This means you only need a single data pin and a clock pin on your microcontroller to access all these device, all you need to do is apply enough clock pulses to get the data to/from all the devices.

The open drain means that the output can only pull low, pull to ground. It cannot by itself make a high signal, it needs help for that from an external resistor. The transistor pulls the output low and when the transistor is 'released' the resistor will pull the output up to Vcc.







simulate this circuit

The advantage of open drain (or open collector when using a BJT) is that you can tie multiple outputs together without problems.