States And Modes 6 - Allen-Bradley 1779-KFMR User Manual

Chapter 6
HDLC Data Link Layer
States and Modes
6 12
A node may be busy transmitting a long data frame while it is receiving a
series of short data frames. After the node finishes transmitting the long
data frame, it could then acknowledge the complete series of received data
frames by transmitting a response frame with N(R) set equal to the current
R value. However, since the variables and sequence numbers can only
have values of from 0 through 7, the number of sequentially numbered
data frames that a node can have outstanding (unacknowledged) at any
given time must never exceed 7.
Poll/Final (P/F) Bit
The poll/final (P/F) bit serves a function in both command frames and
response frames. In command frames, this bit is the P bit. In response
frames, it is the F bit. Setting the P bit to 1 in a command frame solicits a
response frame with the F bit set to 1 from the other node at the earliest
opportunity.
Only one frame with a P bit set to 1 may be outstanding at a given time.
Before a node can issue another frame with a P bit set to 1, it must receive
a response frame with the F bit set to 1 or it must time out. If the node
does not receive a valid response frame within a timeout, it can retransmit
the command frame with the P bit set to 1 for error recovery.
A P bit set to 1 and F bit set to 1 are always exchanged as a pair; for every
poll there is one final response, and the next poll must not be issued until
the previous poll has had a final response or until the response timer
expires. Therefore, the N(R) contained in a frame with a P or F bit set to
1 can be used to detect data frame sequence errors. While detecting data
frame sequence errors, this capability can indicate the data frame
sequence number with which to begin retransmission. This capability is
referred to as checkpointing.
While in the information-transfer state, the computer shall examine the
N(R) of any received data or supervisory frame with the F bit set to 1. If
this N(R) does not acknowledge all data frames the computer had
transmitted prior to and including the last command frame it sent with the
P bit set to 1, it shall initiate appropriate error recovery procedures.
In all cases, the N(R) of a correctly received data or supervisory frame
shall acknowledge previously transmitted data frames through N(R)—1.
One node transmits response frames to the other node based on previous
receipt of a polling command frame. In certain cases, a node can also