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The information in this document is subject to change  without  notice
and  should  not  be  construed  as  a  comitment by Digital Equipment
Corporation.  Digital Equipment Corporation assumes no  responsibility
for and errors that may appear in this document.

The software described in this document is furnished under  a  license
and  may  be  used or copied only in accordance with the terms of such

Copyright (C) 1977,1978 by Digital Equipment Corporation

The following are trademarks of Digital Equipment Corporation:

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          ASSIST-11      RTS-8          ITPS-10
DN92.MEM                                                        Page 2


The purpose of this document is to outline the various  parts  of  the
DC72NP  coding  which were modified to develop the DN92.  Parts of the
document are probably disjointed giving too much or too little  detail
on  the  various  changes  that  were  made.   However,  it  might  be
beneficial to have this generalized outline when looking at  the  DN92
source  file.   This  document  contains  the  DDCMP  and  NCL message
protocols.  It briefly outlines the  methods  used  to  implement  SET
HOST,  STATION  CONTROL  messages,  and modem support.  Since The DN92
Installation Guide describes the ROM'S operating procedures,  the  ROM
loader is not discussed in this document.

1.1  DN92 Components

The DN92 is based on a PDP8A processor with 16K core.  It has  a  VT52
as  a  console terminal, one synchronous line, and 1K ROM for downline
loading.  OPTIONS INCLUDE:  one line printer, (either an  LP05  300LPM
printer  or  an LA180), a card reader, and a maximum of 16 TTY'S.  The
asynchronous lines on the DN92 are  driven  by  the  KL8A  multiplexor
which  drives 4 TTY lines per module.  This module also provides modem
support for the TTY lines.  One TTY line per KL8A module provides full
modem  support,  and  the  other  3  TTY  lines  provide partial modem
support.  The DN92 also provides a driver for the LA180 printer  using
the parallel interface on the PDP8A DKC8-AA I/O option board.


The DN92 was developed to replace the DC72NP remote station.  The DN92
is  based on the DC72NP software and is still a sequential node in the
network.  It assumes its  NCL  messages  are  sent  to  it  in  proper
sequential order and does not ACK, NAK, or REP NCL messages.  However,
the DN92 has  implemented  NCL  CONNECT  messages  and  the  SET  HOST
command.   If a TTY line gets disconnected from a host, it will try to
connect to some host when input is received on the line.  The DN92 has
implemented  station  control  messages so locations in a running node
can be monitored using DDT92.  It has a 1K ROM  which  in  conjunction
with  NETLDR  allows the station to be downline loaded.  The DN92 also
has the following features:

     -Local dump  to  line  printer  or  console  included  in  system
      software (Starting Address 201)
     -Station Control messages implemented in system software so DDT92
      can be used to monitor a running DN92 station
     -SET TTY WIDTH implemented
     -SET HOST implemented
     -Downline loading ROM
DN92.MEM                                                        Page 3


Image and ASCII mode on TTY  lines  are  not  working  properly.   The
problem occurs when the two modes are alternating frequently in a data

3.1  Core Utilization and Buffering

The synchronous line in the DN92 is set up to receive in field  2  and
to  transmit  from  field 1.  The buffer area is divided into a linked
list of 8-12 bit word chunks and is located in field 2.  The  original
DC72NP  code  assumed  all data, DDB tables, and chunks resided in the
same field (usually field 1).  In order  to  increase  the  number  of
chunks available for buffering data, the DN92 moved its chunk field to
field 2.  The synchronous line's receive field was also moved to field
2.  The synchronous line's transmitter buffers were left in field 1 as
well as the DDB table, literal strings, line printer VFU  table,  card
reader  table, and CRC constant table.  The instruction which sets the
data field to field 2  is  "RCKFLD"(RECEIVE  AND  CHUNK  FIELD).   The
instruction    which   sets   the   data   field   to   field   1   is
"TABFLD"(TRANSMITTER AND TABLE FIELD).  All the transmitter logic i.e.
the  code  to  format a message to be transmitted over the synchronous
line is now located in field 1.  It is all the code which is called by
"XSTART"  and  is executed whenever the synchronous line is idle.  Due
to the addressing structure of  the  PDP8  CPU,  the  easiest  way  to
transpose  these  instructions  from field 0 where they resided in the
DC72NP code to field 1 was to  duplicate  or  use  field  0's  page  0
symbols.   The  PAL10 assembler does not differentiate between field 0
and field 1 addresses.  Therefore, field 1 page 0 is left blank in the
listing.   However,  all the instructions in field 1 use the TEMP area
in the same way as the instructions in field 0.  For example,  "TEMP1"
is a location both in page 0 of field 0 and field 1.  When the code in
field 1 actually needs a value stored in a location in field  0  which
is  not  a  temporary  value, it changes its data field to field 0 and
uses indirect addressing via a page 0 literal to get the needed value.
It then changes back to the default data field 1.


The DN92 is a sequential node in the network.  There is only one  path
from  it  to the network.  Its synchronous line must be connected to a
PDP11 based DC75NP, DAS85, DN87, OR  DN87S  front  end;   or  a  DAS80
series  remote station.  The PDP11 based node is a non-sequential node
and insures  that  the  DN92  receives  its  NCL  messages  in  proper
sequential  order.  The DN92 assumes that its NCL numbering is correct
and doesn't keep track of the NCL message numbers.   It  inserts  fill
characters (0) in the NCL header and lets the PDP11-based node put the
proper numbers into the NCL header.  The DN92 does not issue  NCL-ACK,
NCL-NAK,  OR  NCL-REP  messages.   Unlike  the  DC72NP,  it does issue
DN92.MEM                                                        Page 4


The following two sections define the DDCMP and NCL message formats.

5.1  DDCMP Messages (all but  DATA  are  preceded  by  synchronization

DATA --     SOH  CC1    CC2   MSG#  NMSG  A0  BCC1  n*DATA   BCC2
ACK --      ENQ  <001>  FILL  MSG#  FILL  A0  BCC1
NAK --      ENQ  <002>  RNAK  MSG#  FILL  A0  BCC1
REP --      ENQ  <003>  FILL  FILL  NLST  A0  BCC1
RESET* --   ENQ  <004>  FILL  FILL  NNXT  A0  BCC1
RESACK* --  ENQ  <005>  FILL  NEXP  FILL  A0  BCC1
STRT --     ENQ  <006>  FILL  FILL  NBEG  A0  BCC1
STACK --    ENQ  <007>  FILL  NREC  NXMT  A0  BCC1

BOOT --     DLE  CC1    CC2   <000> <000> A0  BCC1  BOOTDATA BCC2

     "n" =the number of data bytes, a 16-bit quantity made up  of  CC1
          and CC2.
     A0  =1 (Station number;  always one for point to point.)
     ADDR=4 byte field containing the address for the core-image  data
          being loaded or dumped.
     BCC1=16 bits of BCC computed on the first 6 bytes of the message.
     BCC2=16 bits of the BCC computed on the "n" data bytes.
     BNUM=2 byte field containing number of bytes to be dumped.
     CC1 =the low order 8 bits of the  character  count  of  the  data
     CC2 =the high order 8 bits of the character  count  of  the  data
          portion.   The  two  high order bits of this byte are really
          flags for the multi-point case, but will always be zero  for
          the point-to-point case.
     DLE =220    (This  is  the   starting   character   for   station
          management messages.)
     ENQ =005    (This  is  the   starting   character   for   control
     FILL=0 (Filler;  is checked and must be zero.)
     IDAT="n" bytes of image data, which the station will put  at  the
          address contained in ADDR.
     MSG#=number of the last good message received (implies ACK of all
          lower numbered messages).
     NBEG=first  message  number  this  station  will  transmit  after
          startup is completed.
     NEXP=message number expected to be sent next(usually  NNXT  field
          of REP message).
     NLST=number of last transmitted data message.
     NMSG=the number of this message.
     NNXT=next numbered message to be transmitted (i.e. lowest message
          that has not been acked).
     NREC=next message number for reception (usually NBEG field of the
          STRT message).
     RNAK=Reason for negative acknowledgement:
          1=Header BCC incorrect
          2=Data BCC incorrect
DN92.MEM                                                        Page 5

          3=The last REP message received indicates  we  lost  one  or
          more messages.
          10=Buffer space temporarily unavailable
          11=Receive overrrun (data lost)
          20=Data message is too long
          21=Header format error (e.g. non-zero fill)
     SNAM=software system defined data identifying  which  program  to
     SNUM=a sequential numbering of successive boot messages.
     SOH =201    (This is the starting character for data messages.)

     BOOTDATA will be one of the following formats:
          BOOT           SNA    <000>
          EXAMINE        SNA    <001>   <adr1> <adr2>
          DEPOSIT        SNA    <002>   <adr1> <data>
          GO TO          SNA    <003>   <adr>
          CLEAR          SNA    <004>   <adr1> <adr2>
          DEBUG          SNA    <005>
          ACCEPT         DNA    <011>   <adr>
          EXAMINE DATA   DNA    <012>   <adr>  <data>
          REJECT         DNA    <013>
          REQUEST BOOT   DNA    <014>   <type> <serial> <description>
          REQUEST LOAD   DNA    <015>   <type> <serial> <description>

     DESCRIPTION=extensible Ascii;  text which describes program to be
          loaded, usually a file description.
     DNA=extensible binary, node number the bootstrap  message  should
          be routed to.  Zero means default.
     SERIAL=extensible binary;  the serial number for the  node  being
     SNA=extensible binary;  the node  number  of  the  station  which
          originated the bootstrap message.
     type=extensible binary;  code for the  type  of  node  requesting

                    1=DC71 (PDP8I with DP01).
                    2=DC72 (PDP8E with DP8E).
                    3= (PDP11/40 with DU11).
                    4=DAS82 (PDP11/40 with DQ11).
5.2  NCL Formats

unnumbered control -- NCT  DNA  SNA  NCA  NCN  OPD
numbered control --   NCT  DNA  SNA  NCA  NCN  0    CM
DATA --               NCT  DNA  SNA  NCA  NCN  DLA  DEVCTL

     DEVCTL=device control.  (see 04.3)
     DLA=destination message link address,  i.e. the  index  into  the
          node's   connection   database.   Extensible  binary  field,
          maximum of 12 bits, zero is illegal.
     DNA=destination NNM
     NCA=Network Control Ack;  last network message received ok.
     NCN=Network Control message Number.  One byte binary field.
     NCT=network control message type and flags, extensible field.
          bits 0-2=type field
                 0=data message
DN92.MEM                                                        Page 6

                 4=start.  OPD is NNM SNM SID.
                 5=stack.  OPD is NNM SNM SID.
                 6=node id.  OPD is NNM SNM SID.
          bit 3=SNA and DNA present.
          bit 4=trace
          bit 5=interrupt msg(i.e.  don't adjust data request count)
          bit 6=Non-sequential node.
          bit 7=extensible bit
     NNM=node name, a binary extensible field,  maximum  of  12  bits,
          identifying  node.   Zero  means  next node over synchronous
     OPD=optional data.
     SID=software identification, extensible ASCII with two subfields:
          1)  name and version of operating system and DEMOS software,
          2) creation date.
     SNA=source NNM.
     SNM=station name is an extensible ASCII field.

CM = one of the following:

CONNECT --         CNT  <001>  DLA   SLA   DPN   SPN   MML  FEA
DISCONNECT --      CNT  <002>  DLA   SLA   RSN
NEIGHBOURS --      CNT  <003>  (NNM  LVL)
REQ CONFIG --      CNT  <004>
DATA REQUEST --    CNT  <006>  DLA   DRQ

     CNT=count of remaining bytes in message.
     DCM=data code and mode:
          b3=Hollerith(CDR only)
          b4=DEC image (CDR only)
          b6=compressed format
     DLA= (defined above).
     DPN=destination PN.
     DVT=device specific attributes:
        =attributes for card reader:
                 0=don't care
                 2=between 300 and 600
            ,b2=mark sense
            ,b3=hdw EOF required
            ,b4=suppress EOF card detection
        =attributes for line printer:
          c1,b0+b1=speed(see DCD)
            ,b2=lower case req
DN92.MEM                                                        Page 7

            ,b3=remov.  char set req
            ,b4=multi-part paper req
            ,b5=12 chan skipping req
          c2,b0+b1=skip requirements
                 0=don't care
                 1=changeable from handler
                 2=changeable at site
                 3=changeable but don't care how
            ,b2=req overprint
            ,b3+b4=6/8 lines/inch
            ,b5=changeable form width
        =attributes for teletypes:
          c1,b0=modem control
            ,b2=handler can set baud rates
            ,b5=auto dial line.
     FEA=features:  DCM+RLN+DVT
     LVL=link value is a one-byte binary value used to  determine  the
          perferred  path.   (Preferred path is that whose sum of link
          values is lowest.)
     MML=maximum NCL message  length  for  connection,  including  NCL
          header, but not including DDCMP.
     OBJ=object type for process:
          0=tty handler
          2=card reader
          3=line printer
          4=paper tape reader
          5=paper tape punch
          7=magnetic tape
          11=task (job)
          12=rdx, data entry terminal.
     OPD=optional data (extensible characters).
     PID=process identification.  For devices this  is  an  extensible
          binary  field, 177 means default choice, 0 - n means unit #.
          For tasks this is a single extensible ASCII  string  usually
          name and qualifier (e.g.  UIC or PPN).
     PN =process name, having 2 parts:  1) OBJ, 2) PID.
          0=normal disconnection
          1=object type not available
          2=too many connects to node
          3=too many connects to process
          4=process does not exist at this node
          10=reassign, next ext field is dest node number
     SLA=source  message  link  address.   Extensible  binary   field,
          maximum of 12 bits, zero is illegal.
     SPN=source PN.
     STC=station control.
DN92.MEM                                                        Page 8



In order to implement SET HOST the DN92 had to set up a  table  called
NEITAB to save the nodes in the network and to indicate which ones had
an MCR handler.   It  adds  entries  to  this  table  when  a  REQUEST
CONFIGURATION  message is received and the node is not currently found
in the table.  If the node is added to the table  two  bits  are  set;
(1)  400  to  issue  a  CONFIGURATION message, and (2) 4000 to issue a
REQUEST CONFIGURATION.  If the node is already in the table  only  the
configuration  bit  (400)  is  set.  Deletions are made from the table
when a NEIGHBORS message is received and the node is no  longer  found
in  the  current  neighbor's  message.   If a CONFIGURATION message is
received, the bit to issue a REQUEST CONFIGURATION (4000)is  set  off.
Bit  (1000)  is  set  if  the node has an MCR handler.  When a REQUEST
CONFIGURATION message is sent the 4000 bit is set off.  Also, when the
CONFIGURATION message is sent, the 400 bit is set off.



When the DN92 receives a DISCONNECT message it checks for a reason  of
10  i.e.  to see if SET HOST is the reason for the disconnect.  If the
reason is 10 the DN92 gets the node number to which it is to  issue  a
connect and places it in the TTY line's DDB table entry called DEVRCN.
It sets the connect bit (2000) off in the DEVSTS entry  and  sets  the
disconnect  confirm  bit (1000) on in DEVSTS.  If the reason is not 10
and the entry at DEVRCN is 0, a disconnect was caused  by  some  other
reason  (This  should  never happen on a tty line!!) In any event, the
code frees the connect table entries (i.e.  0  to  CTRLTB,  RCVKRD  to
RCVDSP table, and connect bit 2000 off in DEVSTS, 1000 bit on to issue
confirmation).  If a DISCONNECT message  is  received  and  DEVRCN  is
nonzero,  the  code assumes the CONNECT issued to that node failed and
the next entry in the NEITAB which has an MCR is stored in the  DEVRCN
entry.  The connect bit and the confirmation bit (1000) are set on.

6.2.2  CONNECT

If a DN92 receives a CONNECT message with a DLA which is  nonzero,  it
assumes  that  it  is  a  CONNECT  CONFIRM.   It  uses  the  DLA  as a
displacement into its SLATAB.  The next word in  the  message  is  the
displacement  into the 10's tables.  It saves this value at the SLATAB
table entry.   It  calculates  the  address  of  the  DEVSTS  for  the
connection, sets the connect bit (2000) on, accept bit off (1000), and
need to send status bit on (1).  The DATA  REQUESTS  are  zeroed,  the
DDCMP status word is zeroed, and the DEVRCN is zeroed.
DN92.MEM                                                        Page 9


If DEVRCN is zero the DN92 will type "TTY NOT CONNECTED"  and  try  to
find  an  MCR  handler  in  the  NEITAB.  The node number found in the
NEITAB is stored at DEVRCN.  The connect confirm bits (3000)  are  set
in  DEVSTS  so  the transmit logic will issue a CONNECT.  If DEVRCN is
nonzero, it assumes a CONNECT has been issued to the  node  in  DEVRCN
and  is  waiting for a CONNECT CONFIRM.  In this case, it simply types


If the disconnect confirm bits (1000) are set,  the  DN92  issues  the
DISCONNECT  CONFIRM.   If  the  DEVRCN  entry  is nonzero, it sets the
connect confirm bits on (3000) so a connect will be issued on the next

If the status bits are set for a CONNECT CONFIRM (3000)  the  software
checks  for  DEVRCN  = 0.  If DEVRCN is zero the DN92 issues a CONNECT
CONFIRM as was done in the DC72NP.  However, if DEVRCN is nonzero,  it
means  a  CONNECT  is to be issued to the node number found in DEVRCN.
First the entry at DEVRNN is checked to see if the line is restricted.
If  the  line  is restricted the DN92 will only issue a CONNECT to the
restricted node number.  The DN92 then checks to be sure the  link  to
the   connect   tables  in  the  DN92  is  set  up  correctly.   After
establishing a proper entry into the connect tables, it zeroes the SLA
table  entry,  stores RCVTTY into the RCVDSP table, and stores the the
link to the connect tables at the CTRLTB table entry.  A check is made
to see if the DEVRCN entry is still in the NEITAB.  If it is no longer
in the table, a zero is stored at DEVRCN and RCVKRD is stored  in  the
RCVDSP  table entry.  The 3000 bits are set off in the DEVSTS word and
no CONNECT message is sent.  If the  DEVRCN  entry  is  found  in  the
NEITAB, a CONNECT message is set up to be transmitted.


To implement modem support on the DN92, two  status  bits  had  to  be
added  to  the DEVDDC word.  Since the PDP8 has a 12 bit word size and
the 12 bits used were already defined, two extra bits  were  added  to
the  DDCMP  status and stored in the DEVDSL entry in the DDB.  The 100
bit is CARRIER/RING status and the 40  bit  is  DATA  TERMINAL  READY.
Since 3 of the lines on the KL8A module have hardware DTR always high,
it was decided to answer the phone locally on line 3 by asserting  the
DTR  when  a  ring occurs.  The ring is set on in the DDCMP status and
sent to the host.  The host should then respond with the DTR on in the
DDCMP  status  word.   Once  every second the DN92 checks all data set
lines (DEVDSL entry has 4000 bit on) to see if the CARRIER is OK.   If
the CARRIER goes away for six seconds, a status message is sent to the
host with the CARRIER status bit off and on line 3 the hardware DTR is
set  off.   Once  per  second  logic is also used to turn the software
CARRIER bit on when the hardware CARRIER bit first comes on.
DN92.MEM                                                       Page 10


The DN92 has implemented  EXAM,  DEPOSIT,  and  GOTO  station  control
messages.   A  table  (BTTAB) was set up to allow five station control
messages to be queued.  However, only one GOTO can be queued.  If  the
DN92 receives multiple GOTO'S, it will reject them until the first one
is completely processed.  Each entry  in  this  table  consists  of  6
words.   The  first word is the node number who initiated the message.
The second word is the message type to be sent  as  a  response.   The
next  four  words  are  the beginning and ending addresses.  The field
specification of the address is stored as the software instruction  to
set  the data to the specified field N (i.e.  6201 + N0).  The station
control messages are scheduled to be processed on the transmitter side
after  CONFIGURATION  and  REQUEST  CONFIGURATION  messages.   When an
accept message is built in the tranmit buffer in response  to  a  GOTO
message,  a  flag  has  to  be set so the actual change in the program
counter can be made after the transmission.  This flag is set  in  two
stages.   When  the accept message is built in the transmit buffer the
high order bit of the first word in the message is set on  (ie  4201).
When  SNDMSG is called, it checks the high order bit of the first word
of the message.  If it finds it on, it bumps a field  0  page  0  flag
GFLG.   The  message  is  sent  at  interrupt level.  After a numbered
message is transmitted, the interrupt level code checks GFLG to see if
it is nonzero.  If GFLG is nonzero it transfers control to the address
stored in the GTTAB table.


Various documents pertaining to the DN92 or  its  hardware  components
are available and are listed below:


      3.  KL8-A USERS MANUAL




      7.  PAL10.RNO FILE

      8.  DC72CK.RNO FILE

      9.  DC72NP.RND FILE
DN92.MEM                                                       Page 11

     10.  DAS82.RNO FILE

     11.  DN92.RND (THIS FILE)


[End of DN92.MEM]