MIL-HDBK-1553A1 November 1988

Figure 50-2. Analog Front End

terminal. It does no decoding or checking of the signal. A receiver contains an input filter to remove noiseand a threshold comparator. It produces an output of 1 (i.e., RCV_OUT high) when 1553_BUS_+ is sufficientlygreater than 1553_BUS_—, an output of 0 (i.e., RCV_OUT/ high) when 1553_BUS_— is sufficiently greaterthan 1553_BUS_+, and both differential outputs high (or low, see 50.3.1.2) when there is no sufficiently largesignal on the 1553 bus.

50.3.1.1 Receiver input filtering. The input filter is a key element in determining receiver performance. Thebus signal is corrupted by the addition of noise, often to the point that it is difficult to decode. This noise is mainlydue to reflections from impedance discontinuities (e.g., stubs, in-line connectors, and terminators) in the busnetwork. Another source of noise is electromagnetic interference (EMI) or coupling of signals into the cablefrom other parts of the system. Both of these types of noise are mostly higher in frequency than the 1553Bfundamental frequency of 1 MHz. The purpose of the receiver input filter is to eliminate as much of this noiseas possible while minimizing signal attenuation.

This noise and signal distortion can cause multiple zero-crossings to occur in a bit time (1 µs) and can alsocause a large error in the time of a zero-crossing (a zero-crossing shift error). This could cause the word tobe misinterpreted by the decoder. The decoder would detect this as a Manchester error, which is a bit thatdoes not have opposite values in the two halves of the bit time. One bit could also be distorted sufficiently thatit is decoded as valid but of the wrong value. The decoder would detect this error with the parity bit. In bothof these cases, the decoder knows the word is invalid, and the message transfer fails. It is also possible thatthe signal could be distorted to such an extent that the decoder thinks it is valid, but decodes the wrong value.The purpose of the receiver input filter is to reduce the word error rate by reducing the noise or distortion ofthe signal. Design experience has shown that the specified word error rate cannot be met without filtering onthe input of the receiver.

A 1553B bus operates at a bit rate of 1 MHz. Due to the Manchester encoding of the data, there are frequencycomponents at 250 kHz, 333 kHz, and 500 kHz as well as 1 MHz. This follows from the possible zero-crossing

Section 50 50-6

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