Rockwell Automation 1771-QC , D17716.5.25 SER/B SERVO POS - Manual (Page 167)

Chapter 8
8Ć6
Because the servo positioning assembly relies on bidirectional block
transfer for communication with the PC processor, the time required for
block transfer operations may be critical in certain situations. For
example, operator commands are transmitted to the 1771-M3 controller
via command block transfer. Depending on when a command is issued,
up to four block transfers (two read, two write) may occur before the
1771-M3 controller can act on it.
Another example involves continuous execution of consecutive movesets.
The 1771-M3 controller stores one moveset block on-board. It transfers
individual moves to the 1771-ES expander one at a time. The 1771-ES
expander stores two moves on-board: the current move it is executing and
the next move. After the 1771-M3 controller transfers the last move of a
moveset to the 1771-ES expander, it requests transfer of the moveset
block from the PC processor. This request is transmitted via the status
block. In the worst case, four block transfers (two read, two write) may
occur before the 1771-M3 controller can send the next move to the
1772-ES expander. If the 1771-ES expander completes execution of the
first two moves of a moveset before it receives the next move, an
unintentional dwell may occur in move execution.
Factors involved in block transfer timing include:
system scan time
block length
system I/O configuration
the number of enabled block-transfer instructions in a given program
scan
To calculate worst case block transfer times, assume maximum block
length (64 words write, 14 words read), and that a block transfer
instruction is enabled for each block transfer module in the system in
every program scan.
The following sections describe calculation of worst case block transfer
times for PLC-2/30 remote and local systems, and for the Mini-PLC-2/15
system.
PLCĆ2/30 Remote System
To find the time between block transfers for a given module in a
PLC-2/30 remote system, perform the following steps:
PLCĆ2ĆFamily Block Transfer
Timing