Question 4.3: Determine the VRCs and LRC for the following ASCII-encoded m......
Determine the VRCs and LRC for the following ASCII-encoded message: THE CAT. Use odd parity for the VRCs and even parity for the LRC.
The LRC is 00101111 binary (2F hex), which is the character “/” in ASCII. Therefore, after the LRC character is appended to the message, it would read “THE CAT/.”
The group of characters that comprise a message (i.e., THE CAT) is often called a block or frame of data. Therefore, the bit sequence for the LRC is often called a block check sequence (BCS) or frame check sequence (FCS).
With longitudinal redundancy checking, all messages (regardless of their length) have the same number of error-detection characters. This characteristic alone makes LRC a better choice for systems that typically send long messages.
Historically, LRC detects between 95% and 98% of all transmission errors. LRC will not detect transmission errors when an even number of characters has an error in the same bit position. For example, if b4 in an even number of characters is in error, the LRC is still valid even though multiple transmission errors have occurred.
| Character | T | H | E | sp | C | A | T | LRC |
| Hex | 54 | 48 | 45 | 20 | 43 | 41 | 54 | 2F |
| ASCII code b_0 | 0 | 0 | 1 | 0 | 1 | 1 | 0 | 1 |
| b_1 | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 1 |
| b_2 | 1 | 0 | 1 | 0 | 0 | 0 | 1 | 1 |
| b_3 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 1 |
| b_4 | 1 | 0 | 0 | 0 | 0 | 0 | 1 | 0 |
| b_5 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 1 |
| b_6 | 1 | 1 | 1 | 0 | 1 | 1 | 1 | 0 |
| Parity bit b_7 (VRC) | 0 | 1 | 0 | 0 | 0 | 1 | 0 | 0 |