Data Link Control Method - Allen-Bradley 1779-KFMR User Manual

Chapter 6
HDLC Data Link Layer

Data Link Control Method

6 2
We define the method of data-link control from several perspectives. On
the RS-422/449 link, the computer and the 1779-KFM interface are
combined stations that form a balanced configuration for two-way
simultaneous operation.
Combined Stations
On the RS-422/449 link, the computer and the 1779-KFM interface are
combined stations. A combined station has balanced link control
capability. A combined station transmits both command frames and
response frames to, and receives both command frames and response
frames from, the other. It maintains one information transmitting ability
to and one information receiving ability from the other combined station.
Balanced Configuration
On the RS-422/449 link, the computer and 1779-KFM form a balanced
configuration. A balanced configuration consists of two combined
stations connected point-to-point, for two-way simultaneous operation.
Both combined stations have compatible data transfer and link-control
capability.
Two Way Simultaneous Operation
On a two-way simultaneous link, two physical circuits connect four
distinct and independent software routines. Figure 6.1 shows these
software routines: Transmitter (XMTR) A and Receiver (RCVR) A are at
one station; Transmitter B and Receiver B are at the other station.
Figure 6.1
Physical Circuits for Two Way Simultaneous Operation
XMTR
A
RCVR
A
Physical Circuit AB
Physical Circuit BA
RCVR
B
XMTR
B