Commands And Responses - Allen-Bradley 1779-KFMR User Manual

Commands and Responses

Data Transfer State
While in the data transfer state (DTS), a node is fully operational and can
transmit and receive data transfer, supervisory, and unnumbered format
frames.
In this state, the nodes are always in the asynchronous balanced mode
(ABM). ABM is a data transfer mode in which the nodes use
asynchronous respond opportunity (see "Asynchronous Respond
Opportunity"). You select this mode with a SABM command. A node
may transmit command frames at any time; therefore, the ABM definition
only describes and applies to the response-frame transmitting and
command-frame receiving capability of the node.
Each frame contains either a command, a response, or both. Both
commands and responses are further broken down into data transfer,
supervisory, or unnumbered format. The following is a list of the
link-level commands and responses used in the 1779-KFM interface:
Format Commands
Data Transfer Data
Supervisory RR Receive Ready
RNR Receive Not Ready
REJ Reject
Unnumbered SABM Set Asynchronous Balanced
Mode
DISC Disconnet
Data Transfer Format
The function of the data transfer command/response is to efficiently
transfer sequentially numbered frames containing a data field. The
encoding of the control field in the data transfer format is as follows:
Chapter 6
HDLC Data Link Layer
Responses
Data
RR Receive Ready
RNR Receive Not Ready
REJ Reject
UA Unnumbered
Acknowledgment
DM Disconnected Mode
FRMR Frame Reject
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