Allen-Bradley 1779-KFMR User Manual page 75

Chapter 6
HDLC Data Link Layer
6 6
Address Field
The address field contains the link-level address on the RS-422/449 link.
The addresses of the two nodes on this link are:
Address
1
3
The address in this field denotes the receiver on this link which is
receiving and responding to a link-level command. Therefore, a link-level
command received by a receiver and the response frame the receiver
transmits in reaction to that command have the same address. The
address contained in each address field is as follows:
Address
1 Command Received by the 1779 KFM Interface
1 Response Transmitted by the 1779 KFM Interface
3 Command Received by the Computer
3 Response Transmitted by the Computer
Control Field
The control field contains a link-level command or a response to a
link-level command. The control field may also contain sequence
numbers. A link-level command in the control field instructs the receiver
to perform a specific link-level operation. For example, a link-level
command might be to receive the accompanying data field. Do not
confuse these link-level commands with the application-level command
and reply messages packeted within the link-level data field.
Data Field
The data field may contain any sequence of bits except the flag sequence.
The transmission of data that contains the same sequence of bits as the
flag sequence is accomplished by a zero-bit insertion procedure described
in "Zero-Bit Insertion." The HDLC data-link layer does not encode or
decode the data field; it receives the data field from and passes on the data
field to the preceding layer.
Node
1779 KFM Interface
Computer
Type of Frame