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Oscilloscopes in
Aerospace/Defense
Debugging MIL-STD 1553 serial buses

Agilent's InfiniiVision 3000 and 4000 X-Series
oscilloscopes provide MIL-STD 1553 triggering and
decoding, as well as eye-diagram mask test capability
to accelerate debug of your MIL-STD 1553 buses.

The differential 1-Mbps MIL-STD 1553 serial bus is widely
used today in a broad range of aerospace/defense systems
including aircraft avionics equipment, space-based systems,
as well as many ground-based military vehicles. Due to
the harsh electrical environment of military equipment,
testing the quality of received and transmitted signals
is very important. The primary measurement tool that
engineers and technicians use today to test the signal The differential probing point for this particular
integrity of serial buses is an oscilloscope. But capturing measurement was at the input of Remote Terminal #2. As
and measuring the electrical characteristics of MIL-STD expected, words received from the Bus Controller (BC) have
1553 signals have been a difficult and tedious process using reduced amplitudes due to transformer coupling. In addition,
conventional analog or digital oscilloscopes. Setting up a the shapes of each receive bit from the BC have been
scope to trigger and synchronize on specific transmitted or distorted -- probably due a long and lossy transmission
received words often requires an external synchronization line within the aircraft. In this case, the scope revealed a
signal or guessing at a specific trigger hold-off setting. And Manchester encoding error (indicated by MANCH text color-
then to determine the message information of captured coded in red) that occurred during the 2nd Command Word
and displayed communication packets/words requires a transmitted by the BC. With this information provided by
commonly used visual "bit-counting" technique, which is the scope, it should now be much easier for the avionics
slow and prone to errors. engineer or technician to troubleshoot the root cause of this
error.
Although MIL-STD 1553 bus monitors/protocol analyzers
can provide the high-level protocol layer information
regarding transfer of data, they tell you nothing about signal
integrity. The answer to this dilemma (scope versus bus
monitor) is to use a scope that is also able to intelligently
trigger on and decode MIL-STD 1553 signals.

Figure 1 shows the display of an Agilent InfiniiVision
X-Series scope capturing and decoding a MIL-STD 1553
RT-to-RT message transfer. All Command, Status, and Data
Words are decoded and displayed in a time-correlated
trace below the captured waveform. In addition, the scope
displays a tabular list of decoded data in the upper half of
the scope's display. Figure 1: Decoding an RT-to-RT message on an Agilent
InfiniiVision X-Series oscilloscope.
In addition to providing decoded words time-correlated to Agilent's InfiniiVision 3000 and 4000 X-Series
captured waveforms, another useful tool to verify the signal Oscilloscopes
integrity of a MIL-STD 1553 bus is to perform an eye-diagram
mask test. Eye-diagram testing is used in a broad range of If you are in the market today to purchase your next
today's serial bus applications. An eye-diagram is basically oscilloscope, Agilent Technologies' 3000 and 4000 X-Series
an overlay of all bits captured by the scope to show when oscilloscopes come in various bandwidth models ranging
bits are valid and not valid. This provides a composite from 100 MHz up to 1.5 GHz. These scopes come with
picture of the overall quality of a system's physical layer a standard 3-year warranty, as well as an industry-first
characteristics, which includes amplitude variations possibly 2-year recommended calibration cycle. When purchased
due to transmission line affects, reflections, system noise, with the DSOX3AERO/DSOX4AERO option, these scopes
over-shoot, ringing, signal edge timing, and jitter. provide MIL-STD 1553 and ARINC 429 serial triggering and
decoding. Eye-diagram mask testing of your MIL-STD 1553
Figure 2 shows a MIL-STD 1553 eye-diagram mask test bus is also available with the DSOX3MASK/DSOX4MASK
at the input of a remote terminal. The scope repetitively mask test option. Agilent provides various MIL-STD 1553
captures and overlays all 17 Manchester-encoded bits of mask files that can be downloaded at no charge for testing
every Command Word transmitted by the Bus Controller. both transformer-coupled systems as well as direct-coupled
Note that since the 3-bit sync field of each word does not systems. For probing your differential MIL-STD 1553 bus,
adhere to Manchester encoding, these bits are not included Agilent recommends the N2791A 25-MHz differential active
in the eye-diagram display. probe.

Figure 2: Testing the signal integrity of a differential
MIL-STD 1553 serial bus using eye-diagram mask testing.

Although a typical eye-diagram based on NRZ encoded data
consists of just one "eye", a MIL-STD 1553 eye-diagram is
quite unique in that it actually consists of two "eyes"; one To learn more about Agilent's InfiniiVision 3000 and 4000
for each half of every Manchester encoded bit. A proper X-Series oscilloscopes and mixed signal oscilloscopes,
MIL-STD 1553 eye-diagram should show just rising and go to: www.agilent.com/find/infiniivision
falling transitions in the middle of each bit (center-screen).
But in this example we can see that the scope captured a
shifted half-bit with a slow rising edge that crossed through
the "keep-out" mask region causing mask test failures. This
is most likely the bit that induced the decoded Manchester
encoding error shown earlier in Figure 1.
Product specifications and descriptions in this document
The diamond-shaped masks that define the waveform "keep- subject to change without notice.
out" area of the MIL-STD 1553 mask shown in Figure 2 are
based on published MIL-STD 1553 minimum input amplitude