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GR716-DS-UM, May 2019, Version 1.29
189
www.cobham.com/gaisler
GR716
23.6
Bus Monitor Operation
23.6.1 Overview
The Bus Monitor (BM) can be enabled by itself, or in parallel to the BC or RT. The BM acts as a pas-
sive logging device, writing received data with time stamps to a ring buffer.
23.6.2 Filtering
The Bus Monitor can also support filtering. This is an optional feature, software can check for this by
testing whether the BM filter registers are writable.
Transfers can be filtered per RT address and per subaddress or mode code, and the filter conditions are
logically AND:ed. If all bits of the three filter registers and bits 2-3 of the control register are set to
’1’, the BM core will log all words that are received on the bus.
In order to filter on subaddress/mode code, the BM has logic to track 1553 words belonging to the
same message. All 10 message types are supported. If an unexpected word appears, the filter logic
will restart. Data words not appearing to belong to any message can be logged by setting a bit in the
control register.
The filter logic can be manually restarted by setting the BM enable bit low and then back to high. This
feature is mainly to improve testability of the BM itself.
23.6.3 No-response handling
In the MIL-STD-1553B protocol, a command word for a mode code using indicator 0 or a regular
transfer to subaddress 8 has the same structure as a legal status word. Therefore ambiguity can arise
when the subaddress or mode code filters are used, an RT is not responding on a subaddress, and the
BC then commands the same RT again on subaddress 8 or mode code indicator 0 on the same bus.
This can lead to the second command word being interpreted as a status word and filtered out.
The BM can use the instrumentation bit and reserved bits to disambiguate, which means that this case
will never occur when subaddresses 1-7, 9-30 and mode code indicator 31 are used. Also, this case
does not occur when the subaddress/mode code filters are unused and only the RT address filter is
used.
23.6.4 Log entry format
Each log entry is two 32-bit words.
Table 184.
GR1553B BM Log entry word 0 (offset 0x00)
31
30
24
23
0
1
Reserved
TIME
31
Always written as 1
30 : 24
Reserved - Mask out on read for forward compatibility
23 : 0
Time tag (TIME)
Table 185.
GR1553B BM Log entry word 1 (offset 0x04)
31
30
20
19
18
17
16
15
0
0
Reserved
BUS
WST
WTP
WD
31
Always written as 0
30 : 20
Reserved - Mask out on read for forward compatibility
19
Receive data bus (BUS) - 0:A, 1:B
18 : 17
Word status (WST) - 00=word OK, 01=Manchester error, 10=Parity error
16
Word type (WTP) - 0:Data, 1:Command/status
15 : 0
Word data (WD)