Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 86)

Publication 1785-UM012D-EN-P - July 2005
6-14 Communicating with Remote I/O
As shown in the previous figure, the controller has the following storage areas
for block-transfers:
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.
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.
The controller places a block-transfer request directly into the
active buffer only if: a buffer is available and no block-transfers to
the slot is in the queue.
waiting queues store block-transfer requests that cannot be placed into the active
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
Maximum Number of
Active Buffers
Per Remote I/O Channel
PLC-5/60, -5/60L, -5/80, -5/80E 23
PLC-5/40, -5/40L, -5/40E 31
PLC-5/30 39
PLC-5/20, -5/20E 43
PLC-5/11 43
Placing the controller in program
mode, cancels block-transfers in
the active buffers and in the
waiting queues.