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E1, E2, error detection code characters. |
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|  MIL-D-70789A (AR)
DESCRIPTION OF SYMBOLS
a. F, flag. A fixed sequence to indicate start or end
of message (01111110), 8 bits.
b.
C,
command. Binary code character defining required
DRU response. Detailed descriptions of each command are found in
Appendices A, B, & C. The command word consists of 5 bits. This
character also contains a message sequence number which is 3
bits.
MESSAGE
COMMAND
SEQ. NO.
L
M
S
S
B
B
XXXXX
XXX
43210
765
A message sequence number is assigned to each command by the
prime system device generating the command. The message
identifier shall contain the same message sequence number in the
response to a specific command. The complete message identifier
can be used by the prime system to correlate responses with
requests.
There may be 0 to 35 data
D1, 2 . . . . N, data.
c.
characters per input command or output message. The number and
contents of data characters are defined in Appendices A through
1).
d. E1, E2, error detection code characters. The error
detection code shall be a 16 bit Cyclic Redundancy Check (GRC)
compatible with the synchronous data link control (SDLC) protocol
as implemented in the Zilog 8030 serial communication controller
(CRC-CCITT polynomial). Each message sent by the DRU shall
contain the CRC code computed for the information being sent.
Each command received by the DRU shall be tested for correct
reception. If an error is detected, the DRU shall ignore the
command, issue a STATUS DATA message indicating an Alert, and set
ALERT DATA character D5 bit 0.
The DRU shall send its STATUS
e. S1, S2, STATUS word.
with every output message. The STATUS word contains 16 bits.
Each bit indicates the presence or absence of a particular mode
or condition. Additional STATUS information (S3 and S4) can be
obtained by requesting the EXPANDED STATUS DATA message. 30.5
explains all of the STATUS conditions.
31
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