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IEN: 139 J. Haverty

                                                            BBN
                                                     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"
     configuration
   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. 1 -

Hosts as Imps April 1980

HISTORY

   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

  1. 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

  1. 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

free.

   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.

HOSTS AND IMPS

   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

  1. 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

specifications.

   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,

  1. 5 -

Hosts as Imps April 1980

etc.

   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.

CONCLUSION

   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

  1. 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

  1. 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.

SOLUTION(s)?

   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.

  1. 8 -
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