Rockwell Automation 1771-QC , D17716.5.25 SER/B SERVO POS - Manual (Page 181)

Chapter 8
8Ć20
Consider an additional I/O Scanner Module (cat. no. 1775-S4A, -S4B)
if you cannot otherwise reduce the block transfer times to meet your
timing requirements.
During a write handshake, the processor also can transfer write data;
and during a read handshake, the processor also can transfer read data.
Special Considerations
When using one 1775-S4A I/O scanner with thumbwheel switchset to 1,
only part of its data handling capacity is available for 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.
Block-Transfer Errors
Once enabled, a block-transfer instruction 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:
rack, group, and module numbers do not match the location of the
installed module
you entered a file length greater than 64 words
you did not create the data file, or the address that you entered
does not match the file you created
If the read and write error bits are on at the same time, 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.