BC MESSAGE BLOCK FORMATS AND BC CONTROL
WORD
BC-to-RT Transfer
Control Word
RT-to-BC Transfer
Control Word
Receive Command Word
Data Word #1
Transmit Command Word
In BC mode, the BU-65552, BU-65551 and BU-65550 support all
MIL-STD-1553 message formats. For each 1553 message for-
mat, the BU-65552, BU-65551 and BU-65550 mandate a specif-
ic sequence of words within the BC Message Block. This
includes locations for the Control, Command and (transmitted)
Data Words that are to be read from RAM by the BC protocol
logic. In addition, subsequent contiguous locations must be allo-
cated for storage of received Loopback, RT Status and Data
Words. FIGURE 9 illustrates the organization of the BC message
blocks for the various MIL-STD-1553 message formats. Note that
for all of the message formats, the BC Control Word is located in
the first location of the message block.
Transmit Command
Looped Back
Data Word #2
Status Received
Data Word #1
Data Word #2
.
.
.
Last Data Word
Last Data Word Looped Back
Status Received
.
.
.
Last Data Word
Tx Mode Code;
With Data
Mode Code;
No Data
RT-to-RT Transfer
Control Word
For each of the BC Message Block formats, the first word in the
block is the BC Control Word. The BC Control Word is not trans-
mitted on the 1553 bus. Instead, it contains bits that select the
active bus and message format, enable off-line self-test, masking
of Status Word bits, enable retries and interrupts, and specify
MIL-STD-1553A or -1553B error handling. The bit mapping and
definitions of the BC Control Word are illustrated in TABLE 9.
Control Word
Control Word
Receive Command
Tx Mode Command
Mode Command
Transmit Command
Mode Command
Looped Back
Mode Command
Looped Back
Transmit Command
Looped Back
Status Received
Data Word
Status Received
Tx RT Status Word
Data #1
Data #2
The BC Control Word is followed by the Command Word to be
transmitted, and subsequently by a second Command Word (for
an RT-to-RT transfer), followed by Data Words to be transmitted
(for Receive commands). The location after the last word to be
transmitted is reserved for the Loopback Word. The Loopback
Word is an on-line self-test feature. The subsequent locations
after the Loopback Word are reserved for received Status Words
and Data Words (for Transmit commands).
.
.
.
Last Data
Rx RT Status Word
Rx Mode Code;
With Data
Broadcast
Control Word
Broadcast Command
Data #1
RT-to-RTs (Broadcast)
Transfer
Control Word
AUTOMATIC RETRIES
Control Word
Rx Broadcast Command
Tx Command
Rx Mode Command
The BU-65552, BU-65551 and BU-65550 BC implements auto-
matic message retries. When enabled, retries will occur, follow-
ing response timeout or format error conditions. As additional
options, retries may be enabled when the Message Error Status
Word bit is set by a 1553A RT or following a "Status Set" condi-
tion. For a failed message, either one or two message retries will
occur, the bus channel (same or alternate) is independently pro-
grammable for the first and second retry attempts. Retries may
be enabled or disabled on an individual message basis.
Data Word
Data #2
Tx Command
Looped Back
.
.
.
Data Word
Looped Back
Status Received
Tx RT Status Word
Data #1
Last Data
Last Data
Data #2
Looped Back
.
.
.
BC INTERRUPTS
Last Data
BC interrupts may be enabled by the Interrupt Mask Register for
Stack Rollover, Retry, End-of-Message (global), End-of-
Message (in conjunction with the BC Control Word for individual
messages), response timeout, message error, end of BC frame,
and Status Set conditions. The definition of "Status Set" is pro-
grammable on an individual message basis by means of the BC
Control Word. This allows for masking ("care/don't care") for the
individual RT Status Word bits.
Broadcast Mode Code;
No Data
Broadcast Mode Code;
With Data
Control Word
Control Word
Broadcast Mode Command
Data Word
Broadcast Mode Command
Broadcast Mode Command
Looped Back
Data Word Looped Back
FIGURE 9. BC MESSAGE BLOCK FORMATS
17
ETC [ ETC ]
