Rockwell Automation 6008-SV2R VMEbus remote I/O Scanner - Manual (Page 171)

Chapter
Programming the Scanner
8
8Ć37
/* map channel & establish pointers */
map_channel(chan);
csa
= (csa_data far *) chan–>ControlStatusArea;
gda = (unsigned short far *) chan–>GeneralDataArea;
/* check for semaphore */
if ((csa–>semaphore & SEMAPHORE) == 0x0000) {
/* no block transfer has completed */
unmap_channel(chan);
return –1;
}
/* get block transfer parameters */
block–>cnfstat
= csa–>cnfstat;
block–>cmd = csa–>cmd;
block–>addr = csa–>addr;
block–>tag = csa–>tag;
block–>datalen = csa–>datalen;
/* if read */
if (csa–>cmd == CMD_OSBTREAD) {
/* copy block data */
for (i = 0; i < block–>datalen; i++) {
block–>data[i] = gda[i];
}
}
/* release semaphore */
unlock_gda(chan);
unmap_channel(chan);
return block–>cnfstat;
}
Continuous BlockĆTransfers
Continuous block-transfer operations are similar to single block-transfer
operations. They send and retrieve data the same way. The continuous
block-transfer offers a way to continuously poll an adapter and reduce
programming overhead.
While running a continuous block-transfer, the scanner prevents the
scanner and a VME processor from simultaneously updating the same
block table entry. Your application must resolve possible contention
among multiple master VME processors simultaneously accessing the
same block table entry.
Important: Your application should read block updates as highly critical
sections of code. The application should have the master processor acquire
a lock on the entry, copy the read/write data, and release the lock without
pausing, sleeping, polling, or handling interrupts. As long as the master
processor holds the lock, the scanner cannot update the block.