David Harper wrote:

I guess on a non-related note, how exactly does equipment on the 1553
bus manage communications to prevent them from overlapping / "stepping
on" each other? Is there something in the data sentence once a unit
begins transmitting that puts all other equipment on "hold" until it
is done sending data? Wouldn't there be interrupts needed for
critical equipment to override less critical sentences?

Collisions don't occur because the bus operates like a classroom where
the "pupils" (remote terminals) never speak unless the "teacher" (bus
controller) calls on them. It does so by transmitting a command word
with the RT's address. The command word also contains a transmit bit
in the high state and the number of words to transmit (32 words max).
The RT transmits the words, followed by a status word, then goes quiet
until it receives another transmit command.

There's no way for a remote terminal to bypass this protocol when it's
desperate for attention. The standard suggests such emergencies could
be handled with a dedicated interrrupt line to the bus controller.


Tony wrote:

The 1553B bus is, by definition, dual redundant. Each
bus has an A and B side. Data is transmitted on one
or the other - never both at the same time.

That's true for Air Force avionics, but the standard has no blanket
requirement for redundancy, does it? MIL-STD-1553B (1978) says, "If
redundant data buses are used, the requirements as specified in the
following shall apply..."

In Notice 2 (1986) a requirement for redundancy was added:

"30.2 Application. Section 30 of this appendix shall apply to all
dual standby redundant applications for the Army, Navy, and Air Force.
All Air Force aircraft internal avionics aplications shall be dual
standby redundant, except where safety critical or flight critical
requirments dictate a higher level of redundancy."

I infer from that paragraph that applications other than "Air Force
aircraft internal avionics" can use single buses.


By the way, the 1553B standard was titled "Aircraft Internal Time
Division Command/Response Multiplex Data Bus" until Notice 2 changed
it to "Digital Time Division Command/Response Multiplex Data Bus". The
language was slightly revised so it no longer sounded like the
standard was only for aircraft.

--

Paul Hirose
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