Parity Generator

Every serial data link from a USB hub to a UART module relies on a parity generator to catch single-bit transmission errors. The 74180 and 74AS280 ICs perform this function at TTL speeds inside industrial controllers and embedded systems.

Core Concept

A parity generator appends one extra bit to a data word so that the total count of 1s in the transmitted word meets a chosen parity convention. Even parity means the total number of 1s including the parity bit is even. Odd parity means the total is odd.

The circuit is built from an XOR tree. XOR outputs 1 only when an odd number of its inputs are 1. For a 3-bit word A, B, C, the even parity bit is P = A ⊕ B ⊕ C. If the data already has even parity, P = 0. If it has odd parity, P = 1 to make the total even.

The 74AS280 handles 9-bit words and operates from a 5 V supply. Its propagation delay is 8 ns typical. Fan-out is 20 TTL loads. It provides both even and odd parity outputs simultaneously, making it useful at both the transmitter and receiver.

Boolean Expression

The even parity bit for an n-bit word is P = D₀ ⊕ D₁ ⊕ D₂ ⊕ … ⊕ Dₙ₋₁. This is a cascade of XOR operations. For n = 4 data bits: P = D₀ ⊕ D₁ ⊕ D₂ ⊕ D₃. The XOR function counts 1s modulo 2. P = 0 when an even count of inputs are 1, and P = 1 when an odd count are 1. For odd parity, invert the output: P_odd = NOT(D₀ ⊕ D₁ ⊕ D₂ ⊕ D₃).

Given:
4-bit data word D3 D2 D1 D0 = 1 0 1 1
Required: even parity bit P

Formula / Rule:
P = D3 XOR D2 XOR D1 XOR D0

Step by step:
Step 1: D3 XOR D2 = 1 XOR 0 = 1
Step 2: result XOR D1 = 1 XOR 1 = 0
Step 3: result XOR D0 = 0 XOR 1 = 1

Final Answer:
P = 1
Transmitted word: 1 0 1 1 1  (five 1s — wait, that is odd)
Re-check: data has three 1s (odd count), so P = 1 makes total four 1s (even). Correct.

Exam Tip: Anna University questions often swap even and odd parity without warning. Remember: for even parity, P = XOR of all data bits. For odd parity, P = XNOR (complement of XOR). Also, the 74AS280 gives both outputs at once — do not confuse the Σ(even) and Σ(odd) output pins. The parity bit is appended, not inserted at a fixed position, so bit ordering matters in multi-choice questions.

Key Properties

• IC: 74AS280 (TTL), CD4070 XOR-based designs for CMOS
• Propagation delay: 8 ns typical for 74AS280 at 5 V
• Supply voltage: 4.5 V to 5.5 V (TTL); 3 V to 18 V for CD4030 CMOS XOR
• Fan-out: 20 TTL loads for 74AS280
• Power dissipation: 245 mW maximum for 74AS280 at full speed
• Both even and odd parity outputs available simultaneously on 74AS280
• Cascade multiple ICs for word widths beyond 9 bits by XORing the parity outputs

Quick Revision

• Parity generator adds one bit; parity checker verifies the received word
• Even parity bit = XOR of all data bits
• Odd parity bit = XNOR of all data bits (or NOT of XOR)
• XOR chain: P = D0 ⊕ D1 ⊕ … ⊕ Dn-1
• 74AS280 handles 9 data bits, propagation delay 8 ns
• Parity detects only single-bit errors; two-bit errors go undetected
• Cascade 74AS280 ICs by XORing their parity outputs for wider data buses
• Exam trap: students often invert the parity bit for even vs odd without recomputing — always recount the 1s in the final word to verify