Aim MIL-STD-1553 - Page 265

MIL-STD-1553 Reference Manual
251
Chapter 7 – Bus Controller Functions
Note: At broadcast transfers, the response timeout is used to check that no
status word is responded. Thus, the Intermessage Gap Time in Standard
Gap Mode shall be greater than the response timeout, to guarantee
proper operation.
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In the Fast Intermessage Gap Mode, the Bus Controller generates Intermessage Gap
Times down to 4 µs. In this mode, the value of ‘gap’ is limited to 6bits. The nominal
Intermessage Gap Time is calculated as:
T
IG
= 4s + (‘gap’ * 0.25s) ( 1 Mbit Transmission Mode)
Therefore, Intermessage Gap times between 4µs up to 19.75µs can be achieved in this
mode. This mode can be used for RT validation and testing, but shall not be used
during normal operation since line busy and word count high checks are disabled on the
BC. Also worst case operation is not guaranteed if this mode is combined with Multi RT,
monitor and Mailbox Operation.
The Fast Intermessage Gap Mode shall only be used for transfer instructions which are
followed by another transfer instruction. In case of a transfer error, the Fast Intermessage
Gap Mode is switched off for this transfer and a default Intermessage gap of app. 11s is
generated.
Note: The idle time of the physical bus is 2.0µs less than this time due to the
gap measurement definition of MIL-STD-1553B.
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Status Word Exception Mask
Bit 15
Bit 14
Bit 13
Bit 12
Bit 11
Bit 10
Bit 9
Bit 8
reserved (0)
MERR
INSTR
SREQ
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
reserved (0)
BRCV
BUSY
SUB
DBCA
TERM
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