User's Manual

ROCKWELL COLLINS

COMPONENT MAINTENANCE MANUAL with IPL

TDR-94, PART NO 622-9352

7). Therefore, a reply sequence with all pulses present constitutes a code 7777.

The X pulse is not defined in ATCRBS replies. This combination of four digits,

each ranging from 0 through 7, makes 4096 (212) different codes possible which

explains the label 4096 code.

and F2 but spaced 21 μs. The intervening pulses, A1 through D4 (except for D1),

make up the coded reply. The coding scheme is defined by ARINC 572, commonly

referred to as the Gillham code, and provides encoded altitude data in the range of

-1000 to 127 000 feet, to the nearest 100 feet. Recall that in mode A the individual

bits assume a binar y value which combines into four, three-bit groups, each group

for m ing an octal digit. In mode C the individual pulses are combined into three

groups. The A, B, and D (except for bit D1) pulses are grouped into two four-bit

groups to encode the altitude to the nearest 500 feet. The C pulses provide the

100-feet deviation (above or below) from the 500-feet level. The mode C encoding

scheme is described in Figure 8/GRAPHIC 34-50-96-99B-075-A01. Notice that the

bit patter n does not follow a conventional binary progression. This scheme was

designed to provide a sequence in which only one bit changes at a time as the

altitude increases or decreases.

(d) The upper part of t he table consists of a matr ix of numbers arranged into rows

and columns. These numbers can be referred to as segment numbers. To find

the altitude to the nearest 500 feet, represented by a given bit pattern, you must

first find the segment that corresponds to that bit sequence. Multiply the segment

number by 500 and subtract 1000 (or subtract 2 from the segment number and

then multiply by 500).

(e) The 100-feet altitude is not as easy. To determ ine the 100-feet level, it is first

necessary to decide whether the 500-feet segment is odd or even. Knowing that,

you add or subtract 0, 100, or 200 feet to or from the 500-feet level depending on

the C bit sequence as shown in the lower portion of the table.

Transponder 4096 Reply Code, Signal Format

Figure 7/GRAPHIC 34-50-96-99B-074-A01

34-50-96

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May 18/06