IEN: 139                                               J. Haverty
                                                       April 1980

                             HOSTs as IMPs

                              Jack Haverty

                      Bolt Beranek and Newman Inc.

     The following discussion is an extract from a report for one of the

ARPA projects at BBN.  The relevant details of the  system configuration

include the following:

     o a PDP-11 with IMP11A interface is used to communicate with

       an LSI-11 using a Collins "1822 interface"

     o the  electrical   connection   uses   the   "distant-host"


     This report was originally issued in October  1979.   It  is  being

reissued  now  as  an IEN after some recent inquiries which indicate the

problem may be more widespread than was previously thought.

                              - 1 -

Hosts as Imps                                          April 1980


     We've  been  having an enormous amount of trouble (more than usual)

since  early  summer getting the LSI-11 and PDP-11  to  talk reliably on

the 1822 lines.   When  the  problems  were  investigated,  the   IMPTST

program also reported errors on the 1822 connections.  This has happened

in  the   past,   and   often   the   problem   would  disappear   after

swapping   some   cables,   or maybe waiting a few hours.  However, this

time the  problem  was  persistent,   which   at  least   gave   us  the

opportunity to figure out what was happening.

     A collection of people have been working on the problem, and  we've

come up with an analysis of the situation.

     The immediate cause of the strange behavior  was  tracked  down  to

large  noise  spikes  induced  in  the  cables.   We  have  noise spikes

every half-cycle on the building power cables.  These are  coupled  into

the  1822 cables by RF coupling.   We  have  seen  as much  as  20 volts

peak-to-peak spikes on the differential signal lines.  This  appears  to

the receivers as common-mode noise.

     Differential  receivers  in  the  various  host   interfaces   have

receivers   which  are spec'ed for good common-mode rejection up to some

maximum input signal level.  For the LSI-11s, the chips  are good up  to

                              - 2 -

Hosts as Imps                                          April 1980

+-15 volts.  IMP11As use chips which are spec'd at +-3 volts.

     For the various "IMP" interfaces, the PTIP interface is good for  a

range  similar  to  that  of  the  BCRs  good  for a similar range.  The

316/516  IMPs provide optical isolation, so they are  good  up  to  some

very  high  value  dependent  on  the  breakdown  of  the insulation.

     The  1822  spec says that host's receivers should expect a  1  volt

peak-to-peak  signal,   centered   around   the   host's   ground.  Host

receivers  also must be able to tolerate at least 2.5 volts  of  common-

mode  noise.   Note  that,  for  the  IMP11As,  DEC  picked chips  which

exactly met the spec, i.e., 2.5 volts noise plus  0.5  volts  signal  in

either  the  plus or minus  direction  requires  an input  range  of  +-

3.0  volts.   The chips in that design have a maximum legal input  range

of  +-3 volts with respect to the PDP-11 ground.  Thus the DEC design is

a legal host interface according to 1822.

     By judicious grounding and other magic, we have  gotten  the  noise

down   to  less  than a volt.  The source was traced to our building UPS

system, which has some bad filter capacitors.  These will  be  replaced,

and  that  is  likely  to  remove the noise  source.  However, 1 volt of

induced noise is not unreasonable, and is also probably  fairly   common

in computer areas.  The UPS problem only makes  the  noise  occur  every

                              - 3 -

Hosts as Imps                                          April 1980

8  milliseconds,   whereas    more  traditional  noise  might  occur  at

intervals of seconds or minutes.

     As of April 1980, the filter capacitors  have been replaced.   This

has  reduced  the  noise  somewhat.   We  have  learned however that UPS

systems can generate significant noise in normal operation.  This  noise

appears  on  the  input  side  fo  the  UPS  system;  the output side is

generally well filtered.  Computer equipment powered from the UPS output

will  not see any significant noise on its power; equipment powered from

non-UPS power will have the noise  present,  but  the  equipment  itself

typically  filters  the  power  enough  so that the logic power is noise


     The problem we have  been  investigating  appears  to  result  from

coupling  between  "1822" cables and power cables carrying non-UPS power

which also carries the noise spikes generated by the UPS system.


     With  the  1 volt noise, the PDP-11 and LSI-11 communication  still

doesn't work properly.  IMPTST reveals  occasional  errors.

     In  tracking  these  problems  down, we delved pretty  deeply  into

all  the pieces of the system, even down to the circuits used within the

                              - 4 -

Hosts as Imps                                          April 1980

driver/receiver chips.  In  the  IMP11A  receiver,  it appears  that  if

the  signal  level  exceeds  3 volts, the logic will get "confused", and

produce an inverted output.  However, with  1 volt  of  noise,  and  0.5

volts of signal, the input should not exceed about 1.5 volts.

     Further tracking along, we looked  at  the  drivers  in   the  LSI-

11.    These  are  single-ended  drivers,  i.e., using a single +5 power

supply.  They produce signals which swing between +0.2  and +2.6  volts.

The  upper  limit  will  depend  on  the  actual value of "+5", resistor

tolerances,  etc.   The  measured  "differential" signal  produced by  a

pair  of  these  drivers keeps one signal line at +0.2, and the other at

+2.4 all the time.  Thus the signal  is a 2.2 volt peak-to-peak  signal,

with a built-in common-mode noise level of 1.2 volts.

     When  an  LSI-11   is   connected   to   an   IMP11A,   the  IMP11A

receivers  see a signal which swings to  +2.6  volts.   Given  0.4 volts

of     noise,     this    drives    the    receivers    beyond     their


     We believe that   this   is   the   reason   behind   the   current

problems  we  are  having with the link.  With the ambient 1 volt noise,

the IMP11As receivers are being driven out of spec all the  time,  which

causes  random errors depending  on  how  the  signals add, other noise,

                              - 5 -

Hosts as Imps                                          April 1980


     We  have also experienced  in  the  last  few  years  a  continuous

problem  of random incompatibilities between the LSI-11s and the IMP11A.

For example, one day a particular LSI-11 might work well to   the   PDP-

11,   and   the   following day it might not.  It seems conceivable that

this is partially caused by the situation I  just  outlined.   The  real

voltage  level on any day might depend on  the building  power  voltage,

temperature,  noise  produced by other equipment in the  vicinity,  etc.

Since  the IMP11A is being driven at the very edge of its working range,

all  the  random  "noise" factors  will  determine  how  well  the  link

works  at  any  time.   We  have  observed  also  that  the  the  system

occasionally  "hangs"  for no  apparent  reason, possibly after  running

for  hours, which may also be related to the aperiodic noise which might

occur   as   air  conditioners   switch,  elevators,  etc.   A   similar

installation  has  also   reported  problems  of   this   nature,   with

unreliable LSI-11/PDP-11 communications using distant-host connections.


     The  consensus of opinion here is that the problem  lies  with  the

LSI-11   1822   interface  characteristics.   The  "1822  spec" dictates

how  to build HOST interfaces, but does not specify  how  to  build  IMP

                              - 6 -

Hosts as Imps                                          April 1980

interfaces.   It  alludes  to  the   fact   that  the  IMP provides  for

things like ground isolation, deskewing, and other features not required

of hosts.

     The 1822 interface is however asymmetric -- it is  NOT   true  that

all  host  1822  interfaces  should  be able to talk to all other hosts'

interfaces.  The host spec defines an interface which  will  allow   any

host  to  talk  to  an IMP;  the IMP interface, which has to talk to ALL

POSSIBLE  host  interfaces,  performs  more   functions  than   a   host

interface  is required to do.  In particular, it is required to  provide

ground  isolation,   and   to   generate   true  differential   signals,

centered   on   the  host  interface's  ground  which  is carried by the

ground-wire in the cable.

     The LSI-11 1822 design seems to adequately follow  the  1822  spec,

in   that   it   provides  a host interface.  The IMP11A also adequately

follows the 1822 spec, as a host interface.

     However, the LSI-11 must act as  an  IMP  --  i.e.,  it   must   be

designed  to  communicate with any "1822 interface" which meets the host

specs.   The  current LSI-11 1822 design does not meet this criterion.

     The root of the problem is probably that most people think  of  the

1822  specification  as  a  symmetric  one,  i.e.,  such that  any  1822

                              - 7 -

Hosts as Imps                                          April 1980

interface  can  talk   to   any   other   1822   interface.    For  most

interface    implementations  and  environments,  this    is    probably

true,   which reinforces the mistake.


     We  are looking at various short-term solutions to see which is the

least  painful  way  to  make  the  system  function  reliably.  In  the

long  run,  we  believe  that  the LSI-11 1822  has  to  be  changed  to

behave  as an IMP.  At the very least, the drivers must generate a +-0.5

volt  signal,  centered   around  ground,   and   provide   for   ground

isolation.   There  may  be other constraints, such  as  timing  issues,

as  well,  which  an  IMP designer  could  help   with.    We  have  not

looked into the LSI-11 1822 design in these areas.

     It is also   worth   noting   that   other   projects   which   use

"pseudo-IMPs"   should   be   examined,  to  see  if  the same situation

exists.  In the LSI-11-IMP11A case, the fact that  the   system   is  on

the  edge of the working range means that it works most of the time, and

is just annoyingly flaky.  Other people  may  have  the same situation.

                              - 8 -