rfc745.txt

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NWG/RFC# 745                                        MDB2 30-MAR-78 43649
JANUS Interface Specifications



Exception:
If the interface is absolutely never intended for use in an environment
where common mode voltage exceeds 7 volts in magnitude, or where the
voltage from either signal wire to the signal ground exceeds 10 volts in
magnitude, then the electrical isolation required in this paragraph may
be suspended, and the corresponding requirements of EIA specification
RS-422 applied in its place.  Such an implementation is explicitly an
exceptional JANUS interface, and is not the standard JANUS interface.

Discussion:
A suggested way to achieve this isolation is an RS-422 receiver chip,
such as the Motorola MC3487 or the Advanced Micro Devices Am26LS32,
followed by an LED driver as needed, followed by an optical isolator
such as the Hewlett-Packard 5082-4360.  The receivers and LED drivers
for all input lines may be powered from one source, but this power must
be floated with respect to ground of the home interface.

2.2.4.  DH Cable Shield Grounding

Requirement:
At each end the cable shield in a distant host connection shall be
connected through a circuit described below to signal ground. The
circuit consists of two components connected in parallel. (1) A 100K,
1/8 watt resistor provides a path to leak off slow accumulations of
static charge.
(2) A .01 mfd, 600 V ceramic capacitor bypasses sharp noise spikes.

Exception:
In cases of severe noise, one end of the shield or the other (but not
both!) may have to be tied directly to ground, sacrificing the symmetry.

Discussion:
Grounding the cable shield only at the host end, as in Report 1822, is
undefined when the interface is symmetrical.  Instead, the circuit above
will be used.

2.2.5.  DH Cable

Requirement:
Cable requirements in EIA specification RS-422 must be followed with
respect to quality and electrical characteristics, and those in Report
1822 with respect to number of conductors.  In particular, at least 10
twisted pairs with impedance of approximately 100 ohms must be supplied.









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NWG/RFC# 745                                        MDB2 30-MAR-78 43649
JANUS Interface Specifications



Discussion:
A suitable cable is PE-39, described in REA Bulletin 345-67. This cable
is similar to that mass produced for telephone cable, which is of good,
uniform quality, and readily available at reasonable cost.  The cable
specified in Report 1822 is not as desirable.  Note the change in
specified characteristic impedance:  Report 1822 specified 120 ohms,
while the JANUS interfaces follow RS-422 with 100 ohms.

2.2.6.  DH Cable Termination

Requirement:
Termination shall be as specified in RS-422, in particular at the
receiver.  Termination as in Report 1822, at the driver, shall NOT be
used.

Discussion:
The source-end termination specified in Report 1822 was to eliminate the
voltage drop caused by the cable's series resistance.  RS-422 explicitly
allows for this sort of signal attenuation as a part of the
specification.

3.  STRONG RECOMMENDATIONS
3.1.  Local Host Signal Levels

Suggested voltage levels for local host drivers and receivers are given
below.  The levels below are a combination of Report 1822 levels for
316/516 and Pluribus machines.  The intent here is to be compatible with
readily available TTL components.  Suggested chips are the 7440 for a
driver and the 7420 for a receiver. Note that signals may go up to 6
volts, which may damage receiving circuits constructed of normal 5-volt
logic.  Such receivers should have a voltage divider on their inputs.

   driver output voltage
        with input = 0:    -   min,  0.35 max  (0.07 typical)
        with input = 1:   3.5  min,  6.0  max  (5.0  typical)
   receiver input voltage
        to assume a binary 0:   0.6  min  (0.9  typical)
        to assume a binary 1:   2.5  max  (1.7  typical)
        maximum input rating:   6.0  max

Cable impedance and termination circuits are covered in Report 1822.
With properly chosen cable and well designed circuits, and with
impedances matched, local host connections may operate considerably
farther than the 30 feet given in Report 1822. Cables as long as 300
feet are in use communicating with ARPANET IMPs.  For example, 300 foot
cables have worked using 7440's as drivers, standard TTL gates as
receivers, cable termination (on all signal lines) of a diode to ground
and a diode to +3 volts, and RG174/U cable.  RG174/U is 50 ohm coax, and
a 100 ohm coax is preferred, to reduce ringing.



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NWG/RFC# 745                                        MDB2 30-MAR-78 43649
JANUS Interface Specifications



3.2.  Use of the Ready Line

It is strongly recommended that the Ready Line provided by the hardware
be used by the software in a manner similar or identical to that
described in Report 1822.  Report 1822 sections 3.2, 4.4 and Appendix B
especially bear on this topic.  In particular, the software design
should provide for the following:

   (1) A ready indicator (relay) which tells the foreign interface that
       the home interface and software are ready to communicate.

   (2) An "error" flip-flop which tells the home software that the
       foreign interface has been not ready.

   (3) NOP messages which are used to purge the communication "pipe"
       after the ready line has "flapped" down and back up.

4.  ADVICE ON DELAYS TO LIMIT BANDWIDTH

It is advisable to include adjustable delays whose function is to limit
the maximum bandwidth of transfers, as discussed in Report 1822.  Only
when the details (such as cable characteristics, memory speed, and
acceptable memory utilization) of a specific application guarantee that
an unregulated transfer rate will be acceptable can these delays be
omitted.  Two delays are involved, one in the sender circuit and one in
the receiver circuit.  The sender delays up to 10 microseconds
(adjustable) from when the foreign interface drops Ready-For-Next-Bit,
before again turning on There's-Your-Bit.  (This is the sum of delays C
and D in Report 1822 Fig. B-1.)  The receiver delays up to 10
microseconds (adjustable) from when the foreign interface asserts
There's-Your-Bit, before again turning on Ready-For-Next-Bit. (This is
the sum of delays A and B in Report 1822 Fig. B-2.) When delivered,
interfaces should have these delays set at approximately the maximum
delay.  The timing is shown below.


















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NWG/RFC# 745                                        MDB2 30-MAR-78 43649
JANUS Interface Specifications



   
                            _______         _______
      sender's TYB  _______!       !_______!       !___

                        _______         _______
      foreign RFNB  ___!       !_______!       !________
                               !<--delay-->!

                            _______         _______
       foreign TYB  _______!       !_______!       !___

                        _______         _______
   receiver's RFNB  ___!       !_______!       !________
                           !<--delay-->!

5.  INTER-OPERABILITY WITH 1822 INTERFACES

Protocol specifications have been chosen which are compatible with
Report 1822.  Actually, the protocol areas discussed above are further
clarification of Report 1822, rather than any change from it.  The
electrical specifications differ only slightly from the 1822 interface.
The local host levels chosen are 1822 compatible.  The potential
difficulties in using a JANUS interface cabled to an 1822 interface
arise with the distant host interface.

The distant host cable for a JANUS interface is 100 ohms nominal
impedance, compared to 120 ohms for the 1822 interface.  This difference
is small enough that most applications will work with either cable, or
even with some 100 ohm cable and some 120 ohm cable.

The 1822 distant host interface does not provide as much electrical
isolation as the standard JANUS distant host interface.  Thus, in cases
of severe common mode noise or ground potential difference, two JANUS
interfaces might operate correctly, but an 1822 interface might
misbehave or burn out.

The JANUS distant host driver yields 2 to 6 volts output, and its
receiver requires 0.2 volts input; the 1822 distant host driver yields
1.0 volt output, and its receiver requires 0.1 volt input. Unless there
is a significant signal loss in the cable, the 1822 driver will drive a
JANUS receiver acceptably.  On the other hand, the maximum input to an
1822 receiver is 4.0 volts.  Thus a JANUS driver might overdrive an 1822
receiver.  The simplest fix for this is to put a (balanced) voltage
divider at the 1822 receiver, or at the JANUS driver.  The divider
should cut down the maximum voltage from 6 volts to 4 volts, or a
reduction of 1/3.






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NWG/RFC# 745                                        MDB2 30-MAR-78 43649
JANUS Interface Specifications



The above differences are relatively minor, so in most applications an
interconnected 1822 interface and a JANUS interface should operate
correctly.  Attention must be paid to the electrical isolation
susceptibility of the 1822, and to its maximum input voltage.

6.  MILITARY COMPATIBILITY

The EIA specification RS-422 chosen as a base for the JANUS interface
distant host electrical characteristics is compatible with military
specification MIL-188-114.

The common mode voltage tolerance of the JANUS interface provides
significant protection against widely varying ground potentials in field
equipment separated by distances of thousands of feet.

7.  REFERENCES

"Specifications for the Interconnection of a Host and an IMP," BBN
Report 1822, revised January 1976; BBN Inc., 50 Moulton St., Cambridge,
Ma. 02138.

"Electrical Characteristics of Balanced Voltage Digital Interface
Circuits, EIA standard RS-422," April 1975; Engineering Dept.,
Electronic Industries Assn., 2001 Eye St., N.W., Washington, D.C.,
20006.

REA bulletin 345-67, Rural Electrification Admin., U.S. Dept. of
Agriculture.  Contains specification for PE-39 cable.
























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