Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 131)

Handshaking
Chapter 5
5Ć2
Read
Data and/or module status is transferred to the processor data table with
each block transfer. When the ASCII module detects a change in its
status, receives new data that is terminated by an end-of-string delimiter
in its input buffer, and/or receives a string greater than the one specified in
IW2, the module toggles the handshaking bit, SW1(15). The module also
places data in status word two (SW2). Bit SW1(15) and SW2 accompany
new data. Your program logic should be written to inhibit using the read
data and/or module status until confirmed as new by examining bit
SW1(15) and word SW2. We recommend that you examine word SW2>0
as a new data condition and examine the status of the read block transfer
done bit, BTR(07,17).
Acknowledgment
The module must receive acknowledgment of the read block transfer
(your program toggles handshake bit CW1(15)) before it can transfer new
read data.
Your program should detect that the module acknowledged receipt of the
write block transfer (module toggles handshake bit SW1(16) before your
program enables another write block transfer.
Handshaking Words
Handshaking is communicated by means of command word one and
status word one. Do not allow data files to overlap the addresses of
command and status words. The first two words of read and write block
transfer files are reserved for command and status words. Data files,
which your program moves into the write block transfer file or out of the
read block transfer file, should overlap all but the first two words of the
read or write block transfer files. PLC-2 family controller example: If
the read and write block transfer files start at addresses 400 and 500
respectively, the corresponding data files should start at 402 and 502,
respectively. PLC-3 controller example: If the read and write block
transfer files start at FB003:0000 and FB002:0000 respectively, the
corresponding data files should start at FB003:0002 and FB002:0002.