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! d i g i t a l !   I N T E R O F F I C E  M E M O R A N D U M
To: DECnet-20 Users 
                                       Date: 28 August 1979 
                                       Dept: NCSS
                                       Loc:  MR1-2/H22
                                       Ext:  HOTLINE (5911)
Subj: Modem strapping for Decnet lines 

     When configuring  a  communications  network  one  must  choose
common  carrier switched or leased, or private lines, modems or null
modems, and communications interfaces.  The  configuration  must  be
hardware  and software compatible, fast enough, reliable enough, and
within your budget.  DECnet-20 V3.0 supports only  synchronous  full
duplex  (4  wire) point to point communication links with no dial up
or dial out capabilities.  Listed below are a number  of  techniques
for  connecting  two nodes to support DECnet-20.  Remember that only
the modems and communications interfaces listed in the  current  SPD
are  supported.   All  configurations  require  network profiles and
customer support plans, and any  configuration  using  non  standard
facilities should include PL90 estimates for support.

     1) Directly connected DMC11s.  Two DMC11-AL's with  line  units
can  be connected directly with coaxial or triaxial cable over short
distances.  The cables connecting the 56KB line units can be  up  to
18000  feet.   The cables connecting the 1MB line units can be up to
6000 feet.  Remember that full duplex needs two  cables  to  connect
two DMCs.

     2)  Two  communication  interfaces  can  be  connected   by   a
synchronous  null  modem  or  modem eliminator over short distances.
The distance from the communications interface  to  the  null  modem
must be no more than 50 feet.

     3) Two synchronous modems, each connected to one  communication
interface  can be directly connected by a 4 wire private line.  This
can be as little as 4 wires each a few feet long.   The  synchronous
modems  connected  by  local wires are often called limited distance
modems.  The ICC commlink-II is a LDM.

     4) Two synchronous modems, each connected to one  communication
interface  can  be  connected  by a 4 wire line leased from a common
carrier such as the Bell system.

     5) Two synchronous modems with  dial  backup  capability,  each
connected  to  one  communications interface can be connected by two
DDS calls in the Bell system.  This is not supported.
To: DECnet-20 Users                                           Page 2
Subj: Modem strapping for Decnet lines

     Following are  the  options  needed  for  the  modems  used  in
DECnet-20 connections.


     This device connects two communications interfaces over a short
distance.  The only option normally offered is choice of line speed.
If the speed is not switch selectably  choose  a  speed  within  the
limits of the communications interfaces.  Remember that a null modem
for synchronous communications must provide a clocking signal.   The
Digital H312-A cannot be used;  it has no internal clock.

     COAXIAL or TRIAXIAL CABLE (56 KB or 1000KB depending  upon  the

     When  connecting  two  DMC11-AL/DMC11-MA  or  DMC11-AL/DMC11-MD
communications  interfaces,  only  cables  and connnecting wires are
required.  They can be connected only over short  distances.   These
devices  contain  their  own clock sources.  No other communications
interfaces can be connected without modems or modem eliminators.

     BELL SYSTEM 201C (2400 bits per second EIA only)

     Bell 201C and 208A modems are similar.  The  following  options
are  specific to the 201C.  See also the modem strapping options for
the 208A.

          No automatic calling  unit.   The  Bell  801/Digital  DN11
     configuration for making outgoing calls is not supported.

          Permanent auto answer.  This  doesn't  matter  for  leased
     line  connections but is important in dial backup situations to
     prevent the need for operator intervention at the called site.

          Four  wire  line  interface.   Full  duplex  communication
     requires a 4 wire interface.

          Zero millisecond RTS/CTS delay.  Full duplex communication
     with  constant  carrier  does not do line turn around or in any
     other way wait for the carrier to settle.

     BELL 208A (4800 bits per second EIA only)

     The following modem strapping options are common  to  the  Bell
208A and the Bell 201C.

          AA connected  to  AB.   This  option  connects  protective
     ground (AA) to signal ground (AB).

          Clock source internal.   Clocking  for  data  transfer  is
     provided by the modem.
To: DECnet-20 Users                                           Page 3
Subj: Modem strapping for Decnet lines

          Continuous  carrier.   Full  duplex   operation   requires
     continuous carrier.

          New synch without.  This option is only used in  multidrop

          Alternate voice either way.  This is the customers choice,
     it makes no difference to the communications interface.

          Switched network  backup  either  way.   Switched  network
     backup allows the customer to place two phone calls through the
     dds network to provide a communications path if his lease  line
     is  failing.  If uninterrupted service is important choose this
     option.  With the cooperation of your local Bell salesman  this
     option  can  also  be  used  to  provide 4 wire service without
     purchasing a leased line.  This  would  provide  a  low  volume
     decnet  link  with much cheaper common carrier rates than would
     be possible with a leased line.

     The following options are specific to the Bell 208A only.

          CC indication of analog loopback on.  In analog loop  test
     mode  this  option  enables the data set ready lead (DSR) which
     the DN20 requires before transmission can begin.

          Continuous request to send  in  continuous  carrier  mode.
     Switched  request  to  send  is  only  used in certain mutidrop

          One second holdover at receiver on line dropout  provided.
     This  prevents  loss  of  the connection when carrier drops for
     very short times.  Since DDCMP performs its own error checking,
     any  errors  encountered  during  short  carrier losses will be
     corrected through retransmission.

     ICC COMLINK II (up to 19200 bits per second EIA only)

     No information currently available.


     Note that local DMC's, and EIA KDPs (up to  19.2KB)  have  been
mentioned.  DECnet-20 can also be purchased with a remote DMC (DMC11
AR/FA) which uses the CCITT V.35 electrical standard.  This  is  for
remote operations above 19.2KB.  We currently have no information on
modems or null modems for the  V.35  interface.   (perhaps  the  GTE
L500A-5 modem is used?) The Bell 303 current loop modem can be used,
but this requires a CCITT V.35 to Bell 303  conversion  unit.   Good

     Note also that Digital makes an EIA DMC.  It  is  not  however,
sold  with  DECnet-20.   The KDP interface is superior to the DMC at
speeds up to and including 19.2KB and EIA doesn't  work  well  above
To: DECnet-20 Users                                           Page 4
Subj: Modem strapping for Decnet lines

     As a special bonus here is a copy of the  Pin  definitions  for
the  25  Pin connectors you will find on your modems.  These will be
helpful when you are using a breakout  box  to  try  to  diagnose  a
communications problem.
To: DECnet-20 Users                                                       Page 5
Subj: Modem strapping for Decnet lines

    |        |       |        |                             |                  |
    |   PIN  |  NAME | TO  TO | FUNCTION                    |     CIRCUIT      |
    |        |       | DTE DCE|                             | (CCITT)   (EIA)  |
    |   1    |  FD   |        | FRAME GROUND                |   101    | (AA)  |
    |   2    |  TD   |      > | TRANSMITTED DATA            |   103    | (BA)  |
    |   3    |  RD   |  <     | RECEIVED DATA               |   104    | (BB)  |
    |   4    |  RTS  |      > | REQUEST TO SEND             |   105    | (CA)  |
    |   5    |  CTS  |  <     | CLEAR TO SEND               |   106    | (CB)  |
    |   6    |  DSR  |  <     | DATA SET READY              |   107    | (CC)  |
    |   7    |  SG   |        | SIGNAL GROUND               |   102    | (AB)  |
    |   8    |  DCD  |  <     | DATA CARRIER DETECT         |   109    | (CF)  |
    |   9    |       |  <     | POSITIVE DC TEST VOLTAGE    |          |       |
    |   10   |       |  <     | NEGITIVE DC TEST VOLTAGE    |          |       |
    |   11   |       |        | UNASSIGNED                  |          |       |
    |   12   |  SDCD |  <     | SEC. DATA CARRIER DETECT    |   122    | (SCF) |
    |   13   |  SCTS |  <     | SEC. CLEAR TO SEND          |   121    | (SCB) |
    |   14   |  STD  |      > | SEC. TRANSMITTED DATA       |   118    | (SBA) |
    |   15   |  TC   |  <     | TRANSMITTER CLOCK           |   114    | (DB)  |
    |   16   |  SRD  |  <     | SEC. RECEIVED DATA          |   119    | (SBB) |
    |   17   |  RC   |  <     | RECEIVER CLOCK              |   115    | (DD)  |
    |   18   |       |      > | RECEIVER DIBIT CLOCK        |          |       |
    |   19   |  SRTS |      > | SEC. REQUEST TO SEND        |   120    | (SCA) |
    |   20   |  DTR  |      > | DATA TERMINAL READY         |   108.2  | (CD)  |
    |   21   |  SQ   |  <     | SIGNAL QUALITY DETECT       |   110    | (CG)  |
    |   22   |  RI   |  <     | RING INDICATOR              |   125    | (CE)  |
    |   23   |       |      > | DATA RATE SELECT            |   111/112|(CH/CI)|
    |   24   |  (TC) |      > | EXTERNAL TRANSMITTER CLOCK  |   113    | (DA)  |
    |   25   |       |      > | BUSY                        |          |       |