Aim MIL-STD-1553 - Page 234

MIL-STD-1553 Reference Manual
220
Chapter 7 – Bus Controller Functions
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Three different Gap Modes are available which are described after the following table:
Value
Constant
Description
0
API_BC_GAP_MODE_DELAY
Delay mode / Transfer Wait Time mode
1
API_BC_GAP_MODE_STANDARD
Standard Gap Mode
2
API_BC_GAP_MODE_FAST
Fast Intermessage Gap Mode
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In this mode ‘gap’ (14bit) specifies the time from the start of a transfer to the start of the
following transfer in µs.
Note: If the specified time is shorter than the actual time needed for transfer
transmission, ‘gap’ is ignored and the Bus Controller automatically
generates a minimum intermessage gap of app. 11µs.
This gap mode guarantees deterministic framing independent from RT response times
and provides excellent repeatability of timed bus events.
The nominal Transfer Wait Time is calculated as:
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= ‘gap’ * 1s
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In Standard Gap Mode ‘gap’ (14bit) specifies the wait gap between the end of the current
transfer and start of the following transmitted Command Word. The nominal
Intermessage Gap Time is calculated as:
T
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= ‘gap’ * 1s
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.
Note: The minimum Intermessage gap time achieved using the Standard Gap
Mode is equal to the minimum gap time achieved in the Transfer Wait
Time Mode.
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
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= 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.