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Control Field Format. The control field is formatted differently for the
different types of command/ response functions provided. The formats for
this field are as follows:
NOTE: This sample format is reversed from that described in Reference B in
Appendix C to show the way the bit pattern would reside in the 990
minicomputer.
Figure 5-10 Control Field Format
Control Field Poll/Final Bit. The P/F (Poll/Final) bit is designated “P”
when used with a command frame and is designated “F” when used with a
response frame. The “P” bit is used by the primary to solicit a response from
a secondary when it is set to one. The secondary will respond with the “F”
bit set to one to indicate the end of transmission. Unbalanced configurations
may only have one outstanding command frame with the “P” bit set to one
at any given time. Before the primary can send another command frame
with the “P” bit set to one, it must receive a response frame with the “F” bit
set to one. The only exception to this is if the secondary does not respond to
the primary poll within the system timeout limits.
The information format is used to convey information or data to/from
primary and to/from secondary. The fields N(S) and N(R) are the message
sequence numbers for messages sent (N(S)) and messages received (N(R)).
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