Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 207)

ASCII Module
PLC-2 Family Processors
Appendix A
AĆ39
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
When the scanner encounters a communication fault, it tries twice to
complete the transfer. It sets the error bit after the second unsuccessful
try.