To build an eight-bit adder from two four-bit adders, which connection is required between the low-order and high-order devices?

Introduction / Context:
Cascading smaller adders to form wider adders is a standard digital design technique. Ensuring correct carry propagation between the lower and higher nibbles determines correctness for multi-bit operations.

Given Data / Assumptions:

• Two 4-bit adders are used to create one 8-bit adder.
• The low-order adder handles bits 3..0; the high-order adder handles bits 7..4.
• Each adder provides a carry-out and may accept a carry-in.

Concept / Approach:
Ripple chaining requires that the carry generated by the lower block become the carry-in of the upper block. This preserves arithmetic correctness so that sums exceeding 15 in the low nibble increment the high nibble appropriately.

Step-by-Step Solution:

Verification / Alternative check:
Timing analysis shows a carry chain from LSB to MSB; functional simulation confirms correct results across all input combinations with this connection.

Why Other Options Are Wrong:

• High-order carry-in to ground: Ignores carries from the lower nibble.
• High-order carry-out to ground: Not a valid cascading connection.
• Low-order sum to high-order data input: Data buses are independent; only carry must propagate.

Common Pitfalls:
Leaving Cin_high floating; forgetting to account for initial global Cin for full-adder chains.

Final Answer:
the low-order carry-out to the high-order carry-in