Shift registers — identifying unsupported modes Which of the following is <em>not</em> a standard, meaningful transfer characteristic of a shift register?

Introduction / Context:
Shift registers are sequential circuits that move data one bit per clock along a chain of flip-flops. They are categorized by how data enters (serial or parallel) and how data leaves (serial or parallel). Understanding valid input/output pairings helps you recognize common parts like SISO, SIPO, PISO, and PIPO devices.

Given Data / Assumptions:

• SIPO: serial-in/parallel-out (e.g., 74HC595).
• PISO: parallel-in/serial-out (e.g., 74HC165).
• PIPO: parallel-in/parallel-out (used more as register files/temporary storage).
• SISO: serial-in/serial-out (basic shifter).

Concept / Approach:
A valid label describes one input mode and one output mode. “Serial in / parallel in” describes two input methods without specifying how the data exits, so it isn’t a coherent or standard characterization of a shift-register transfer mode. By contrast, SIPO, PISO, and PIPO describe a clear input-to-output relationship.

Step-by-Step Solution:
List known, standard types: SISO, SIPO, PISO, PIPO.Check each option against these standard categories.Find the option that does not pair input with an output method: “Serial in / parallel in.”Therefore, it is not a valid/standard mode description.

Verification / Alternative check:
Datasheets and textbooks uniformly use SISO, SIPO, PISO, and PIPO nomenclature. There is no conventional “SI/PI” category because that phrase does not define an output behavior.

Why Other Options Are Wrong:

• Serial in / parallel out: SIPO is common for expanding outputs from microcontrollers.
• Parallel in / serial out: PISO is used to read multiple inputs with few pins.
• Parallel in / parallel out: PIPO acts as a register/temporary storage with parallel transfer.

Common Pitfalls:

• Misreading “in/out.” Each standard label must pair one input mode with one output mode.
• Assuming any two descriptors form a valid mode; they must describe input and output, not both inputs.

Final Answer:
Serial in / parallel in