Rockwell Automation 6008-SV2R VMEbus remote I/O Scanner - Manual (Page 50)

Chapter
Communicating with Remote I/O
4
4Ć13
Maintaining Block Integrity
To maintain block integrity within the global RAM, use bit 7 of the
semaphore byte. The semaphore byte is in the control/status area of global
RAM for each channel. See chapters 5–6, depending on the operating
mode you select for the scanner.
At power up, when the scanner is turned on, the semaphore bit is 0 (reset).
When the scanner completes a block-transfer, it tests and sets the
semaphore. If the semaphore was reset, the scanner loads status (either
confirmation or error) into the confirmation status word and optionally
interrupts the VME master processor to notify it of pending status. If the
semaphore was set, the scanner holds the block-transfer information until
the semaphore is reset by a VME master processor.
After receiving the status, the VME master processor must copy the data
from the global RAM and reset the semaphore.
Processing One Block Transfer Request
The order in which the scanner sends block-transfer requests to adapters
depends on the scanner’s scan list. The scan list tells the scanner when to
poll an adapter; an adapter can be in a scan list multiple times.
Figure 4.8
Adapter scan sequence
adapter
adapter
adapter
Rack n
Rack 2
Rack 1
❶ scanner sends discrete output data,
which includes a code identifying
upcoming blockĆtransfer, if any
scanner
remote I/O
buffer
❷ adapter accepts output data and sends
discrete input data, which includes a
status bit that tells the scanner to send
the blockĆtransfer
❸ scanner recognizes status bit and
sends blockĆtransfer
❹ adapter receives blockĆtransfer and
processes the request