34
1553 BUS CONNECTIONS
The isolation transformers should be placed as physically close
as possible to the respective TX/RX pins on the SSRT. In addi-
tion, the distance from the isolation transformers to any connec-
tors or cables leaving the board should be as short as possible.
In addition to limiting the voltage drops in the analog signal
traces when transmitting, reducing the hybrid-to-transformer and
transformer-to-connector spacing serves to minimize crosstalk
from other signals on the board.
The general practice in connecting the stub side of a transformer
(or direct) coupled terminal to an external system connector is to
make use of 78 ohm twisted-pair shielded cable. This minimizes
impedance discontinuities. The decision of whether to isolate or
make connections between the center tap of the isolation trans-
former's secondary, the stub shield, the bus shield, and/or chas-
sis ground must be made on a system basis, as determined by
an analysis of EMI/RFI and lightning considerations.
In most systems, it is specified that the 1553 terminal's input
impedance must be measured at the system connector. This is
despite the fact that the MIL-STD-1553B requirement is for it to
be measured looking directly in from the bus side of the isolation
transformer.
The effect of a relatively long stub cable will be to reduce the
measured impedance (looking in from the bus). In order to keep
the impedance above the required level of 1000 ohms (for trans-
former-coupled stubs), the length of any cable between the 1553
RT and the system connector should be minimized.
"SIMULATED BUS" (LAB BENCH) INTERCON-
NECTIONS
For purposes of software development and system integration, it
is generally not necessary to integrate the required couplers, ter-
minators, etc., that comprise a complete MIL-STD-1553B bus. In
most instances, a simplified electrical configuration will suffice.
The three connection methods illustrated in FIGURE 17 allow
the SSRT to be interfaced over a "simulated bus" to simulation
and test equipment. It is important to note that the termination
resistors indicated are necessary in order to ensure reliable com-
munications between the SSRT and the simulation/test
equipment.
FIGURE 17. "SIMULATED BUS" (LAB BENCH) INTERCONNECTIONS
SSRT
HYBRID
TEST/
SIMULATION
EQUIPMENT
STUB
COUPLING
STUB
COUPLING
ISOLATION
TRANSFORMER
78
1.5W
(A)
SSRT
HYBRID
TEST/
SIMULATION
EQUIPMENT
DIRECT
COUPLING
DIRECT
COUPLING
ISOLATION
TRANSFORMER
39
0.5W
(B)
55
1W
55
1W
55
55
SSRT
HYBRID
TEST/
SIMULATION
EQUIPMENT
STUB
COUPLING
DIRECT
COUPLING
ISOLATION
TRANSFORMER
39
0.5W
(C)
55
1W
55
1W
20
0.5W
20S
0.5W
(A) TRANSFORMER COUPLED TO TRANSFORMER COUPLED
(B) DIRECT COUPLED TO DIRECT COUPLED
(C) DIRECT COUPLED TO TRANSFORMER COUPLED