Should I use Transformer or Direct Coupling?

Any 1553 bus consists of a single twisted-shielded-pair of wires, terminated at each end with proper termination resistors. Connections between terminals and the bus are made through the use of stubs. There are two types of stub, the Direct Coupled (or Short) stub and the Transformer Coupled (or Long) stub.

A Transformer or Long stub can be up to 20 feet in length, and typically connects to a small box or "coupler" which is wired in series with the actual bus. This transformer coupling consists of an external transformer and isolation resistor at the junction to the main 1553 bus to match the stub impedance to the bus. Transformer coupling provides better noise immunity and isolation over long distances.

Direct coupling is often used in the laboratory because it requires no external transformers. A Direct or Short stub can be up to one foot long and is typically created by the use of a "Tee". The bus goes into and out of the Tee while the terminal is connected directly to the third leg of the Tee. Direct coupling may be used when the distance from the isolation transformer inside the device to the data bus does not exceed one-foot. There are 53.6-ohm isolation resistors on the board for direct coupling.

Both methods make use of an internal transformer on the board.

The following figure shows how a single terminal can be connected to a bus controller using direct coupling. The bus is terminated at each end with a terminating resistor, and that the connection from the Tee to the terminal is very short. Typically the Tee is connected directly to the connector on the back of the terminal.

The following figure shows how a single terminal can be connected to a bus controller using transformer coupling. The bus is terminated at each end with a terminating resistor, just as in Figure 1, but now the two Tee's have been replaced with transformer couplers. The connection to the terminals is no longer the length of the Tee but significantly longer, consisting of another cable up to 20 feet in length.

Just because this coupling method is called "Long Stub" doesn’t mean that the stub needs to be 20 feet long. That is just the maximum length specified in the standard.

In both cases, the bus is terminated at each end with a terminating resistor.

Bus coupling (Transformer or Direct) is controlled by jumpers on the board, or by relays that are selected under software control. See the Hardware Installation sections of the "BusTools/1553-API User’s Manual and Hardware Installation Guide" for setting the jumpers (on those boards with jumpers). See the API routine BusTools_SetVoltage() for setting the bus coupling via software control.

It is IMPERATIVE that the bus coupling selection on the board matches the physical bus coupling. Otherwise no data communication will occur.