Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 200)

ASCII Module
PLC-2 Family Processors
Appendix A
AĆ32
Block Transfer with the ASCII Module
Your ladder program must contain read and write handshake logic. This
logic is separate from block transfer routines that use enable and done bits
of block transfer instructions. Handshake logic uses control and status
bits of the ASCII module.
Execution Time
The time required to complete a read or write block transfer depends on
factors that include the number of:
words of user program
active I/O channels on the scanner
I/O chassis entries in the rack list for the channel
I/O channels on the scanner that contain block transfer modules
block transfer modules on the channel (if the I/O chassis containing a
block transfer module appears more than once in the I/O chassis rack
list, count the module once each time the chassis appears in the rack
list)
Typical time required to complete a read or write block transfer depends
on the program scan and the scanner scan as follows:
Time (read or write)= Program scan + 2[Scanner scan]
Program Scan
The program scan is approximately 2.5ms per 1K words of user program
when using a mix of examine on/off and block instructions.
Scanner Scan
The time required for the scanner to complete a read or write block
transfer depends on the number of other block transfer modules on the
same scanner channel that are enabled simultaneously. Use the following
procedure to calculate the time required for the PLC–3 processor to
perform all block transfers on the channel.