Data bus concepts and considerations

The digital data bus MIL-STD-1553B was designed in the early 1970s to replace analog point-to-point wire bundles between electronic instruments. The latest version of the serial local area network (LAN) for military avionics known as MIL-STD-1553B was issued in 1978. Since then, various notices have been published to update the standard and after 30 years the data bus continues to be the most popular militarised network.

The bus has four main elements: (1) a bus controller that manages the information flow; (2) remote terminals that interface one or more simple subsystems to the data bus and respond to commands from the bus controller; (3) the bus monitor that is used for data bus testing; (4) data bus components (bus couplers, cabling, terminators and connectors).

Data is sequentially transmitted and received in a multiplexing scheme over two copper wires from computer to computer at a rate of 1 megabit per second. In most vehicle applications, redundant buses are employed.

The data bus LAN topology encompasses:

• Bus couplers (coupling transformers with fault-isolation resistors);
• Terminators and cabling that includes twinax cable (twisted shielded pair);
• Concentric twinax connectors (with a centre contact and an intermediate cylindrical contact).

Bus couplers are available in various stub configurations including box type and in-line type (used in vehicles where light weight and small size are important).

The purpose of the coupler is to reduce reflections and maintain signal impedance levels. 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 prevent shorting the bus.

In addition to transformers, the bus couplers have built-in fault isolation resistors providing protection for the main bus if there is 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.

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 Ω terminators. This is to minimise 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.

Some couplers have built-in terminators and are generally used at the end of the bus in multicoupler applications. These types of couplers are mainly for vehicle applications as they limit the flexibility of test lab set-ups.

In a lab application, unused stub ports on the coupler need not be terminated since the stubs have a higher impedance than the bus. A high-impedance terminator (1000 to 3000 Ω) may be used in vehicle applications to simulate a future load from an unspecified device. In both cases, an RFI cap over the unused stub is a deterrent to interference and/or dust.

MIL-1553B does not specify the length of the bus. However, the maximum length 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 ns per 300 mm. Thus, the end-to-end 30 m bus would have a 160 ns 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-1553B specifies that the longest stub length is 6 m 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 minimise signal distortion, it is desirable that the stub maintain a 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.

Typically, the cable used to connect the bus and stub devices has a characteristic impedance of 78 Ω at 1 MHz. A two-conductor twisted-pair cable known as twinax is used to connect the bus and stub devices. The insulated pairs are balanced and have an overall shielding braid around the pairs. The twisting of the signal-carrying pairs theoretically cancels any random induced noise caused by the pair.

The two internal dielectric fillers separate the braid from the pairs to minimise the leakage capacitance to ground. The fillers also assist in uniform twisting of the pairs. The 90% braid coverage protects the pair from external noise.

PVC outer jacket cable is suitable for lab use while high-temperature rated outer jacket cable is applicable for vehicle use.

There are several types of connectors used on the bus and at the coupler stubs, the most common of which is the concentric twinax connector. The concentric twinax connector has three bayonet coupling slots/lugs known as TRB type (same envelope size as a coaxial BNC connector).

The centre contact is high (positive) connected to the twinax blue wire and the cylindrical contact is low (negative) connected to the twinax white wire. The body of the connector is bus shield.

Additional mating interfaces include: (1) two-bayonet, (2) four-bayonet and (3) threaded. Because bayonet-type connectors do not require safety wiring and can withstand severe shock and vibration, these are typically preferred over threaded types.

There is a subminiature version of the twinax concentric connector known as TRS type (same envelope size as TPS coaxial connectors). The mating interface options are the same as those for the TRB style above.

Other connector shell options employing concentric twinax contacts include D-subminiature and cylindrical multipin (for example, MIL-DTL-38999 or D38999).

Systems designers must be aware of able compatibility of connectors and availability of components before finalising the design of a data bus system.

MIL-STD-1553B DATA BUS REQUIREMENTS

COMMUNICATIONS LINE

Cable type, two-conductor twisted pair;

Capacitance, 30 Pf/300 mm, max;

Twist, four per 300 mm, min.;

Char. impedance (Zo), 70 to 85 Ω at 1 MHz;

Attenuation, 1.5 dB/30 m at 1 MHz max.;

Bus length, not specified;

Termination, two ends terminated in resistors = (Zo) ±2%;

Shielding, 90% coverage min, 00% dual standby redundant.

CABLE COUPLING

Stub length, up to 6 m (may be exceeded);

Stub voltage, 1-14 V p-p amplitude, line-to-line min. Signal voltage, transformer coupled.

COUPLER TRANSFORMER

Turns ratio, 1.41:1;

Droop, \<20% (1);

Overshoot/ringing, \<+1 V (1);

CMR, \>45 dB at 1 MHz (1);

Fault protection, series resistors = 0.75 Zo ±2%.