BCRT-33
7.5 BC Operational Example (see figure 18 on page 35)
The BCRT is programmed initially to accomplish the
following:
The first Command Block is for a four-word RT-RT transfer
with the BCRT monitoring the transfer and storing the data.
• Auto-retry is enabled on the opposite bus using only one
retry attempt, if the incoming Status Word is received
with the Message Error bit set.
• Wait for a time delay of 400µs before proceeding to the
next Command Block.
• The Data List Pointer contains the address 0400H.
The second Command Block is for a BC-RT transfer of
two words.
• The End of List bit is set in its Control Word.
• The Data List Pointer contains the address 0404H.
• The Polling Enable bit is set and the Polling Compare
Register contains a one in the Subsystem Fail position
(bit 2).
Then:
A. The CPU initializes all the appropriate registers and
Command Blocks, and issues a Start Enable by writing a
“1” to Register 0, bit 0.
B. The BCRT, through executing a DMA cycle, reads the
control word, command words, and the Data List
Pointer. The delay timer starts and message execution
begins by transmitting the receive and transmit
commands stored in the Command Blocks. The BCRT
then waits to receive the status word back from the
transmitting RT.
C. The BCRT receives the RT status word with all status
bits low from the transmitting RT and stores the status
word in Command Block 1. The incoming data words
from the transmitting RT follow. The BCRT stores them
in memory locations 0400H - 0403H.
If the status word indicates that the message cannot be
transmitted (Message Error), the response time-out
clock counts to zero and the allotted message time runs
out. An auto-retry can be initiated if programmed to do
so. Nevertheless, the ME bit in the control word is set.
D. The BCRT receives the status word response from the
receiving RT. The ME bit in the status word is set,
indicating the message is invalid. The BCRT initiates
the auto retry function, (as programmed) on the alternate
bus, re-transmits the command words, receives the
correct status word, and stores the data again in locations
0400H - 0403H. This time the status word response
from the receiving RT indicates the message transfer
is successful.
E. The timer delay between the two successive
transactions counts down another 135 microseconds
before proceeding. This is determined as follows:
The message transaction time is approximately 130
microseconds (the only approximation is due to the
range in status response and intermessage gap times
specified by MIL-STD-1553B). Approximating that
with the retry, the total duration for the two attempts
is 265µs.
F. The BCRT reads the Tail Pointer of Command Block 1
and places it in the Current Command Register. It also
reads the control word, command word, and Data List
Pointer, and the first data word in the second
Command Block.
G. Since this is a BC-RT transfer, the BCRT transmits the
receive command followed by two data words from
locations 0404H - 0405H in memory. The BCRT reads
the second data word from memory while transmitting
the first.
H. The BCRT receives the status response from the RT. In
this case, the status word indicates, by the ME bit being
low, that the message is valid. The status word also has
the Subsystem Fail bit set.
I. The status word is stored in the Command Block. The
BCRT, having encountered the end of the list, halts
message transactions and waits for another start signal.
J. The BCRT asserts a High-Priority Interrupt indicating
the end of the command list. Due to the polling
comparison match, the BCRT also asserts a Standard
Priority Interrupt and logs the event in the Interrupt
Log List.