&KDSWHU
/ &21),*85$7,21
90( *DWHZD\
)LJXUH 2XWSXW RI WKH &RXQWHU
Note that in the example, the PLC will clear the flag bit BEFORE actually reading the
data. If this methodology is followed, the PLC will never miss a data transfer as the
next scan will reflect the fact that new data has been sent by LINK even if a collision
occurs. Also, the upper portion of each address will reference the VME slot number
which, of course, changes from application to application.
Because the flag bits are optional when reading data from LINK, the portion of the
example encircled by the dashed line is all that is required if the processes which follow
are quick. When used, it is more efficient to process the flags as bytes, then gather and
process the data for all eight entities associated with the flag byte. The flags must be
used when the PLC writes data to LINK. In that direction, the flags are not optional.
:$51,1*
Long VME memory instructions may cause the VME processor to
lock out its bus for extended cycles and cause the L5211 VME
Gateway to enter a H-Error state. Slower VME central processing
units may require shorter VME byte instructions. Instructions 8 bytes
long have proven successful, even with slow PLCs. Dividing a long
VME memory instruction into smaller ones on the same rung will
break up the buss accesses and prevent L5211 timeout errors.
