User:Af64writer/Sandbox

Bus Hardware Characteristics
The bus hardware encompasses (1) cabling, (2) bus couplers, (3) terminators and (4) connectors.

Cabling
Although MIL-STD-1553B specifies that the data bus should have characteristic impedance between 70 and 85 ohms, industry has standardized on 78 ohms. Likewise, the industry has generally standardized on the cable known as twinax cable that has a characteristic impedance of 78 ohms.

MIL-STD-1553B does not specify the length of the bus. However, the maximum length of bus is directly related to the gauge of the cable conductor and time delay of the transmitted signal. A smaller conductor attenuates the signal more than a larger conductor. Typical propagation delay for a 1553B cable is 1.6 nanoseconds per foot. Thus, the end-to-end 100-ft. bus would have a 160 nanosecond propagation delay, which is equal to the average rise time of a 1553B signal. According to MIL-HDBK-1553A, when a signal's propagation delay time is more than 50% of the rise or fall time, it is necessary to consider transmission line effects. This delay time is proportional to the distance propagated. Also, consideration must be given to the actual distance between the transmitter and receiver, and the individual waveform characteristics of the transmitters and receivers.

MIL-STD-1553B specifies that the longest stub length is 20 feet for transformer coupled stubs, but can be exceeded. With no stubs attached, the main bus looks like an infinite length transmission line with no disturbing reflections. When a stub is added, the bus is loaded and a mismatch occurs with resulting reflections. The degree of mismatch and signal distortion due to reflections are a function of the impedance presented by the stub and terminal input impedance. To minimize signal distortion, it is desirable that the stub maintain high impedance. This impedance is reflected back to the bus. At the same time, however, the impedance must be kept low so that adequate signal power will be delivered to the receiving end. Therefore, a tradeoff between these conflicting requirements is necessary to achieve the specified signal-to-noise ratio and system error rate performance (for more information, refer to MIL-HDBK-1553A).

Bus Couplers
The purpose of the bus coupler is to reduce reflections, maintain signal impedance levels, and protect the bus in an event of a short circuit. Since direct coupled devices (without couplers) provide no DC isolation or common mode rejection, direct connection to the bus should be avoided. Without couplers, any shorting fault between the device's internal isolation resistors (usually found on the circuit board) and the main bus will cause failure of the entire bus because the device's internal isolation resistors are not sufficient to ensure against shorting out the bus. In addition to transformers, the bus couplers have built-in fault isolation resistors providing protection for the main bus in the event of a short circuit in the stub. All devices, including the bus controller, bus monitor and remote terminal must be connected to the stub ends of the coupler.

Terminators
Both ends of the bus, whether it includes one coupler or a series of couplers connected together, must be terminated (in accordance with MIL-STD-1553B) with 78 ohm terminators. The purpose of termination is to minimize the effects of signal reflections that can cause waveform distortion. If termination is not used, the communications signal can be compromised causing disruption or intermittent communications failures.

Connectors
Although there are potentially many types of connectors that can be used on the bus cable and couplers, the most common type is the concentric twinax connector. The most popular concentric twinax connector has three bayonet coupling slots/lugs (same envelope size as a coaxial BNC connector).