Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 133)

Handshaking
Chapter 5
5Ć4
Table 5.A
Logic Conditions for a Read
If Then
a) SW1(15) ≠ CW1(15)
and
b) SW2>0 and
BTR (07,17) =1
at processor
Program acts on new module status (a, only)
or
Program acts on new data (a and b)
and
Program sets CW1(15)=SW1(15) to acknowledge.
SW1(15)=CW1(15) at
module
Module resets for next transfer of new data
and/or status.
Module sets SW1(15) ≠ CW1(15) to transfer new
status and/or data, and sets SW2>0 to transfer new
data.
A read (only) program requires only read handshaking. A read/write
program requires both read and write handshaking.
Writing Data to the Module
Handshaking in a write operation requires your program to toggle bit 16
in command word one, CW1(16). When the toggled bit status is
transferred with data to the ASCII module, the module processes the data
to the output buffer where it is transferred to the ASCII device. Your
program must contain a one-shot to ensure that CW1(16) is toggled only
once with each transfer of new data.
After the module processes the data, it toggles bit 16 in status word one,
SW1(16), and returns the toggled bit status to the processor to
acknowledge receipt (Figure 4.2 and Table 5.B).