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+Network Working Group                                       M. Wingfield
+Request for Comments: 12  REVISED                         26 August 1969
+
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+                    IMP-HOST INTERFACE FLOW DIAGRAMS
+
+   The following flow diagrams were extracted from the logic diagrams
+   provided in Appendix B of BBN Report No. 1822.  These diagrams
+   indicate the logical sequence of hardware operations which occur
+   within the IMP-HOST interface.  The logic names appearing in the
+   blocks correspond to the logic elements found in Appendix B.
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+Wingfield                                                       [Page 1]
+
+RFC 12              IMP-HOST INTERFACE FLOW DIAGRAMS      26 August 1969
+
+
+IMP to HOST Message
+                       +----------+
+                       |  Start   |
+                       |  Input   |
+                       +----------+
+                             |
+                             V
+                 +------------------------+
+                 | LAST <--0; IMPERR <--0 |
+                 | PAD <--0; IN REQST <--0|
+                 |  COUNTER <--0          |
+                 +------------------------+      +---+
+                             |<----------------- | B |
+                             V                   +---+
+                      +--------------+
+                      | RFNIB <-- 1  |
+                      +--------------+
+                             |<--------------+
+                             V               |
+                            / \              |
+                          /     \            |
+                        / THERE'S \ N        |
+                        \YOUR BIT /----------+
+                          \=1 ? /
+                            \ /
+                             |
+        +------------------->V
+        |                   / \
+   +----+------+    N     /     \
+   | IMP ERROR |<------ /   IMP   \
+   | <--- 1    |        \  READY  /
+   +-----------+          \ ?   /
+                            \ /
+                             | Y
+                             V
+                            / \
+                          /     \
+   +-----------+   Y    /  LAST   \
+   | PAD <-- 1 |<-------\ IMP BIT /
+   +-----------+          \ =1? /
+        |                   \ /                 +---+
+        +------------------->|<-----------------| C |
+                             V                  +---+
+                +-----------------------+
+                |    PULSE SHIFT REG.   |       +---+
+                |   INCREMENT COUNTER   |------>| A |
+                |      RFNTR <-- 0      |       +---+
+                +-----------------------+
+
+
+
+Wingfield                                                       [Page 2]
+
+RFC 12              IMP-HOST INTERFACE FLOW DIAGRAMS      26 August 1969
+
+
+      +---+                +---+
+      | C |                | A |
+      +-+-+                +-+-+
+        ^                    |
+        | Y                  V
+       / \                  / \
+     /     \          N   /WORD \
+   /  PAD=1  \ <--------/ LENGTH  \
+   \    ?    /          \   =1    /
+     \     /              \  ?  /
+       \ /                  \ /
+        | N                  |
+        V                    V
+      +---+       +----------------------+
+      | B |       |     INREQST <--1     |
+      +---+       +----------------------+
+                             |<-----------+
+                             V            |
+                            / \           |
+                          /DATA \         |
+                        /HAS BEEN \_N_____|
+                        \ TAKEN   /
+                          \ ?   /
+                            \ /
+                             V Y
+                  +----------------------+
+                  |    INREQST <--0      |
+                  +----------------------+
+                             V
+                            / \
+                          /     \              +------------+
+                        / LAST = 1\----------->|   END  OF  |
+                        \    ?    /            |   MESSAGE  |
+                          \     /              +------------+
+                            \ /                       ^
+                             | N                      |
+                             V                        |
+                            / \                       |
+                          /     \                     |
+                        / PAD = 1 \-------------------+
+                        \     ?   /
+                          \     /
+                            \ /
+                             | N
+                             V
+   +---+          +---------------------+
+   | B |<---------|    CLEAR COUNTER    |
+   +---+          +---------------------+
+
+
+
+Wingfield                                                       [Page 3]
+
+RFC 12              IMP-HOST INTERFACE FLOW DIAGRAMS      26 August 1969
+
+
+HOST TO IMP MESSAGE
+
+            +--------------+
+            | START OUTPUT |
+            +------+-------+
+                   V
+       +------------------------+
+       | COUNTER <-- 1          |
+       | LSTWD <--0; BTAVL <--0 |
+       +-----------+------------+        +---+
+                   |<--------------------| C |
+                   V                     +---+
+       +-----------+------------+
+       |    OUT REQST <-- 1     |
+       |     CLEAR SHIFT REG.   |
+       +-----------+------------+
+     +------------>|
+     |             V
+     |            / \
+     |      N   /     \
+     |_______ / STROBE  \
+              \DATA WORD/
+                \ =1 ?/
+                  \ /
+                   V  Y
+      +------------+------------+
+      |  LOAD SHIFT  REGISTER   |
+      |  OUTREQST <-- 0         |
+      |  COUNTER  <-- 1         |
+      +-------------------------+
+      +---------->|
+      |           V
+      |          / \
+      |        /     \
+      |N     / RFNBIT  \
+      +----- \   = 1   /
+               \   ? /            +----------+
+                 \ /              |          | N
+                  V   Y           |         / \
+         +--------+--------+      V       /     \
+         |  THERE'S YOUR   |----------->/ RFNBIT  \
+         | HOST BET <-- 1  |            \  = 0    /
+         +-----------------+              \  ?  /
+                                            \ /
+                                             | Y
+                                           +-+-+
+                                           | A |
+                                           +---+
+
+
+
+Wingfield                                                       [Page 4]
+
+RFC 12              IMP-HOST INTERFACE FLOW DIAGRAMS      26 August 1969
+
+
+                               +---+
+                               | A |
+                               +---+
+                                 V
+                  +--------------+-------------+
+                  | THERE'S YOUR HOST BIT <--0 |
+                  |      BTAVL <-- 0           |
+                  +----------------------------+
+                                 V
+                                / \                   / \
+                              / WORD\   Y           /     \   Y
+                            /LENGTH -1\---------->/  LSTWD  \---+
+                            \   = 1   /           \   = 1   /   |
+                              \  ?  /               \  ?  /     |
+                                \ /                   \ /       |
+                                 | N                   | N      |
+                                 V                     |        |
+                                / \                    |        |
+                              / WORD\                  |        V
+           ________________ / LENGTH  \                |  +------------+
+           |                \  = 1    /                |  | LIBIT <- 1 |
+           |                  \  ?  /                  |  +------------+
+           |                    \ /                    V        |
+           |                     |<-----------------------<------+
+           |                     V
+           |            +-----------------+
+           |            | PUSH SHIFT REG. |
+    +-------------+     |INCREMENT COUNTER|
+    | LIBIT <-- 0 |     +-----------------+
+    +-------------+              V
+           |                   +---+
+           |                   | B |
+           V                   +---+
+          / \
+        /     \
+      /  LSTWD  \ Y                          +-------------+
+      \   = 1   /--------------------------->|   END OF    |
+        \  ?  /                              |   MESSAGE   |
+          \ /                                +-------------+
+           | N
+           V
+         +---+
+         | C |
+         +---+
+
+
+         [ This RFC was put into machine readable form for entry ]
+             [ into the online RFC archives by Lorrie Shiota]
+
+
+
+Wingfield                                                       [Page 5]
+
+RFC 12              IMP-HOST INTERFACE FLOW DIAGRAMS      26 August 1969
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+Wingfield                                                       [Page 6]
+