Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 130)

Chapter
5
5Ć1
Handshaking
In this chapter you will read about the use of handshaking to control the
transfer of data from the ASCII module to the PC processor and vice
versa.
The term handshaking refers to a set of software bits that coordinate the
transfer of data between two devices. Handshaking ensures that new data
is neither duplicated nor lost. Your ladder program must contain read and
write handshake logic. This logic is separate from block transfer routines
that use enable and done bits of block transfer instructions. Handshaking
requires the successful completion of both a read and write block transfer
to read new data from the module, or write new data to the module. The
handshake logic uses control and status bits of the ASCII module. New
data is transferred only after correct handshaking is achieved.
Become familiar with the following operations. Refer to the “Complete
Getting Started Program” with rung descriptions for PLC-2 family or
PLC-3 controllers in the appendix.
Write
Data is transferred to the module in each write block transfer. The module
inhibits transmission of data to the ASCII device until one-shot
handshaking is achieved. Then the module transmits data to the ASCII
device in a single transmission. The module will not transmit more data
to the device until the module receives one-shot handshaking with new
data.
When your program writes data to the ASCII module, the program must
toggle the write handshake bit, CW1(16). The status of this bit
accompanies the data. When the module detects the changed status of the
handshake bit, it transmits the data to the ASCII device.
Chapter Objectives
Understanding Handshaking
Fundamentals