Allen-Bradley PLC-5 User Manual page 86

6-14 Communicating with Remote I/O
Maximum Number of
Active Buffers
Per Remote I/O Channel
PLC-5/60, -5/60L, -5/80, -5/80E
PLC-5/40, -5/40L, -5/40E
PLC-5/30
PLC-5/20, -5/20E
PLC-5/11
Placing the controller in program
mode, cancels block-transfers in
the active buffers and in the
waiting queues.
Publication 1785-UM012D-EN-P - July 2005
As shown in the previous figure, the controller has the following storage areas
for block-transfers:
Storage Area Description
active buffers store initialized block-transfer requests for a channel
The adjacent table lists the maximum active buffers for each
enhanced and Ethernet PLC-5 controller.
23
The controller places a block-transfer request directly into the
31
active buffer only if: a buffer is available and no block-transfers to
39
the slot is in the queue.
43
waiting queues store block-transfer requests that cannot be placed into the active
43
buffer because:
all of the channel's active buffers are being used
the slot addressed by the block-transfer is currently processing a
block-transfer
Once a block-transfer to a slot completes, the controller checks the queue to
see if a block-transfer addressed to the slot is waiting. If one exists, the
controller moves it to the active buffer.
Since a controller can request a block-transfer from every slot in a chassis
concurrently, the adapter device chooses the order in which the block-transfers
execute on the chassis. Block-transfer requests are processed differently in
fault routines, selectable timed interrupt routines (STI), and controller input
interrupt routines (PII); see chapters 16, 18, and 19 respectively for more
information.