Envoy ACP/XF Application Programming Manual

Features Common to Bit-Synchronous Protocols
EnvoyACP/XF Application Programming Manual–132179
B-4
Control Field
Frame Format Types
There are three frame formats, specified by the control code (the least-significant bits) of
the control field:
All control fields contain some combination of the following pieces of information:
•
control code
•
Ns (number-sent variable)
•
P/F bit (poll/final bit)
•
Nr (number-received variable)
These standard components of the control field are:
Frame Format Control Code
Type Bit 0 Bit 1
Information (I-frame) 0 x
Supervisory (S-frame) 1 0
Unnumbered command-response
(U-frame)
11
control code This code specifies the frame type, and consists of the low-order
bits of the control field.
Ns Number-sent variable. Ns specifies the link-level sequence
number (modulo-8 or modulo-128) of the particular I-frame in
which it appears. Only I-frames contain Ns.
In modulo-8 sequencing, Ns increments with each successive
frame from 0 through 7, then wraps around to 0.
poll/final (P/F) bit Poll/final bit is one bit. It is the poll bit (P bit) when it is
transmitted by the primary/combined station, and the final bit (F
bit) when transmitted by the secondary/combined station. The
function of the P/F bit depends on the data-link protocol and on
whether the station is a primary or secondary station:
•
For NRM data-link protocol:
•
The primary station sets the P bit to 1 in command
frames to solicit (poll) a response or sequence of
response frames from a secondary station.
•
The secondary station sets the F bit to 1 to indicate the
final frame.