Edgetech PORT LF, Руководство пользователя по оборудованию

Страница: 47 / 79

5-1

SECTION 5: BACS SERIES CODING
BACS’ codes are 16-bits long with 11-bits of information. Each code is specifically referred to as 15, 11
block cyclic code with an overall even parity bit appended to the end. The information bits occupy the first
11-bit positions, the cyclic parity or redundant bits occupy the four positions after the information bits,
and the overall parity bit occupies the last position. This particular code has good mathematical structure.
The parity bits are easily calculated and a minimum Hamming distance of 4-bits exists between any two
codes. The command timing is as follows:
1.
8 bits are sent with a period of roughly 250 ms and a pulse width of 22 ms
2. A word decode interval is 2.718 seconds
3. Following the first decode interval, a 40 ms detection window is open
4. If a detection occurs in the window, a second word decode interval occurs
5. During any of the sample gate intervals, if both "D0" and "D1" are present or if neither one is
present then a command abort will occur and the command will not go through.
From the 11 information bits there are approximately 2,000 unique commands for each tone pair
employed. Commands composed of equal or repeated words are excluded, as are some containing very
few transitions. Since there are six tone pairs available (refer to
T
ABLE
2-4 ), over 12,000 unique commands
exist. Certain ranges of these codes, which are rich in transitions, are designated as ‘guarded’ commands;
these are used for critical functions such as RELEASE.