Rockwell Automation 1771-DCM USER MANUAL 1771-DCM - Manual (Page 46)

Calculating Transfer Time
Chapter 7
7Ć12
Distribute block transfer instructions equally throughout your program.
Place an equal number of non-block transfer rungs between block
transfer rungs.
For large numbers of block transfer instructions, distribute groups of
block transfer rungs equally throughout your program. Place no more
than four block transfer rungs consecutively in one group. Within each
group, condition the next rung using the done bit of the previous block
transfer instruction.
Consider an additional I/O scanner module (cat. no. 1775-S4A) if you
cannot otherwise reduce the block transfer times to meet your timing
requirements.
Special Timing Considerations
When using one 1775-S4A I/O scanner with thumbwheel switch set to 1,
only part of its data handling capacity is available for handling block
transfers. This scanner can store and transfer a maximum of 72 words at
any one time. This scanner can do four block transfer modules across any
of the active channels.
If a block transfer read instruction is enabled but the scanner’s buffer
cannot accept the instruction’s block length (the scanner is processing
other blocks of data), the block transfer instruction must wait for a
subsequent scan when the scanner’s buffer can accept all the words that
the module has to transfer. The same applies for a write block transfer
instruction. We suggest that you add an additional scanner if necessary.
Block Transfer Errors
Once enabled, a block transfer instruction in a PLC-3 ladder program will
set either a done bit or an error bit. The instruction indicates an error
when it illuminates the -(ER)- symbol. Typical block transfer errors occur
when:
You do not correctly enter the instruction:
- The rack, group, and module numbers do not match the location of
the installed module.
- You entered a file length greater than 64.
- You did not create the data file or the address that you entered does
not match the file you created.
Read and write error bits illuminate at the same time when the error
source is the module address entry or the file length entry in the
instruction block.