Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 236)

ASCII Module
For PLCĆ3 Processor
Appendix B
B-14
Special 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, from up to 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.
You have a communication problem
You did not correctly connect the twinaxial cable to the scanner
You did not connect a terminator resistor to each end of the
twinaxial cable