CT2577 Datasheet, PDF (13/41 Pages) Aeroflex Circuit Technology – APPLICATION NOTE 108

completion of the read/write command. The read/write operation must be completed within
the remaining 8 µSec. If a read/write operation starts after the NCMDSTRB strobe has
begun, the NACK will not occur and thus hold off the subsystem for the duration of the DMA
cycle from internal RAM.
During NCMDSTRB, the Command Word is loaded into the Command/Status FIFO stack
and the NEMPTY line goes high. The user can utilize this signal as an indication that some
activity has occurred. The Command/Status FIFO stores up to 32 command words for the
subsystem to review. This allows the processor to only response to the 1553 unit when
something has occurred. Constant polling of the 1553 unit is not required. To reduce
processor intervention even further, the Command/Status FIFO will only store commands
that have associated data with it.
MIL-STD-1760 FEATURES
If the 1760 features are enabled (NENCHK line is held low), the checksum word is automatically
generated and transmitted as the last data word. The subsystem processor does not have to
calculate or load the checksum word into RAM. The SmaRT unit automatically does this in hardware
and transmits the correct checksum data word as the last word out. This reduces subsystem
processor overhead significantly.
SIGNALS THAT INDICATE CHECKSUM FAILURE
For 1760 applications, the STATUS line indicates if a message has failed checksum. The STATUS
line will stay high for a Transmit Command word because there is no associated data words received
for checksum validation. STATUS is sampled on the falling edge of NCMDSTRB. The STATUS line
can be tied to any of the Status Word Bit inputs to set those bits in the event of a checksum fail.
STATUS is an open output line that will set the selected Status Word Bits for the Status Word
response in the current message. This is one of the great features for this product. The subsystem
does not have to verify the checksum in software to detect the error. The SmaRT unit automatically
does this in hardware and the unit is able to flag the error and set the a Status Word Bit on the
CURRENT Status Word response . This minimizes processor overhead and reduces response time
in notifying the Bus Controller that an error has occurred.
For 1760 applications, the NVALCHK signal does not change from it's previous state since a Transmit
Command Word has no incoming data words. NVALCHK is a latched version of the STATUS signal
and is updated only on Receive Commands. It is valid on the falling edge of NCMDSTRB of a
Receive Command and remains stable until the next Receive Command message. A Transmit
Command message will not alter this signal because a Transmit Command does not require an
incoming checksum validation.
T0-15
CMD WD
NCMDSTRB
NEMPTY
NVALCHK
***
8.5 µSec
APPLICATION NOTE #108
PREVIOUS STATE
13
***
Released 9/98