Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 49)

Asynchronous Link Protocols
Chapter 4
4-6
1010 0000 0000 0001
As each additional byte is transmitted, it is included in the value in the
register the same way. After the ETX value is included in the value in the
register and is transmitted, the value in the register is transmitted (right bit
first) as the CRC field.
The receiver also calculates the CRC value and compares it to the
received CRC value to verify the accuracy of the data received.
Two-Way Simultaneous Operation
On a two-way simultaneous link there are only two physical circuits
connecting 4 distinct and independent programs. Referring to the diagram
below, Transmitter A and Receiver B manage the transfer of messages
from Station A to Station B by sending message packets from A to B and
returning responses from B to A. At the same time, Transmitter B and
Receiver A carry out the transfer of messages from Station B to Station A
by sending message packets from B to A and returning responses from A
to B.
Figure 4.2 shows the four independent data paths involved.
Figure 4.2
Data Paths for Two-Way Simultaneous Operation
Transmitter
A
Receiver
B
Path 1
11556
Receiver
A
Transmitter
B
Path 3
Path 2
Path 4
Path 1 carries message codes from A to B; Path 2 carries response codes
from B to A; Path 3 carries message codes from B to A; and Path 4 carries
response codes from A to B.