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NWG/RFC# 745                                        MDB2 30-MAR-78 43649JANUS Interface SpecificationsNetwork Working Group                                     Michael BeelerRequest for Comments 745                                             BBNNIC 43649                                                  30 March 1978PRTN 245                     JANUS Interface Specifications                   (Symmetrical, 1822-like Interface)1.  INTRODUCTION1.1.  MotivationA need arose in the Packet Radio project for specification of aninterface between Packet Radio units and other equipment.  This paper isto meet BBN's responsibility to supply that specification.  It is ourhope that it will find application in other areas as well.1.2.  Historical Relationship to 1822The ARPANET employs a network of switching nodes, called IMPs, toprovide interconnection among user equipment, called hosts.  A uniformmeans of connecting a host to an IMP is specified in BBN Report Number1822.  Consequently, this interface has become known as an 1822interface.As the need to interconnect new types of devices has grown, it hasbecome attractive to implement an 1822-like interface on each end ofpairs of devices which are to communicate.  The devices are thenconnected electrically, and communication can take place in spite ofdifferences in processing speed, word length, signal levels and so forthin the two devices.  A part of Report 1822 reads as follows.   "The technique of transferring information between the Host and the   IMP is identical in each direction; we will, therefore, refer to the   sender and the receiver without specifying the Host or IMP   explicitly."   [BBN Report Number 1822, 12/75 revision, page 4-2.]Unfortunately, Report 1822 does not specify a completely symmetricalinterface.  Although there is a high degree of symmetry, some aspectsare peculiar to the IMP side and some to the host side.  Therefore, twointerfaces constructed to connect to IMPs may not function connected toeach other.  In what follows, the unsymmetrical aspects are respecifiedin a way which will accomplish full interchangeability.The interface specified here is called the JANUS interface, todistinguish it from the Report 1822 interface.                                 - 1 -

NWG/RFC# 745                                        MDB2 30-MAR-78 43649JANUS Interface Specifications1.3.  TerminologyThe terms, "IMP" and "host," are not relevant in the present context.Sections of Report 1822 such asAppendix B are convenientlyre-interpreted by substituting "foreign interface" and "home interface,"respectively.2.  SPECIFICATIONSReport 1822 addresses two aspects of the connection of a host to theARPANET, the hardware requirements and the software protocols.  Sincethe JANUS interface will typically be used in applications other thanconnection to the ARPANET, the higher level software protocols arebeyond the scope of this paper. They are properly addressed bydocumentation specific to each application.  Concern here is only forelectrical specification of the JANUS interface.  The various areaswhich differ from Report 1822 are as follows.2.1.  Low-level ProtocolCertain aspects of the JANUS interface and its operation may beimplemented in hardware, software of a mixture of the two.  We refer tothese aspects as "low-level protocol."  They are to be distinguishedfrom such "high-level protocol" aspects as header definitions and dataformats.2.1.1.  PaddingRequirement:Received messages are padded out to a full word (of the home device'ssize), if necessary, with zeros only.Discussion:A one-bit to mark the end of received data, as IMPs employ, is NOT used.The mark bit has not proved very useful, although the ARPANET IMPs douse it to generate the message length field in the new format header.Rather, counts at one or another level of protocol are generally used,so the complication of a mark bit can be eliminated.  It is the author'simpression that the ARPANET will not implement this aspect ofsymmetrical interfaces, so hosts communicating through the ARPANET willcontinue to see the marker one-bit appended by the source IMP regardlessof whether the hosts have 1822 or JANUS interfaces.2.1.2.  Message LengthRequirement:A JANUS interface must accept messages up to and including 8160 bitslong.                                 - 2 -

NWG/RFC# 745                                        MDB2 30-MAR-78 43649JANUS Interface SpecificationsException:If the interface is absolutely never intended for use inARPANET-compatible applications, this requirement may be relaxed in anyof three ways.  A smaller maximum length may be implemented;  a largermaximum lengthbe implemented; or the maximum length may be so large asto be in practice infinite.Discussion:A JANUS interface may discard messages longer than 8160 bits when usedwith the ARPANET.  This constraint can be enforced in software ratherthan in hardware, if desired.2.1.3.  Four-way HandshakeRequirement:The interface must use the four-way handshake.  That is, the receivermust wait until the incoming There's-Your-Bit drops before turning onReady-For-Next-Bit.Discussion:The two-way handshake, presented as an option in Report 1822, must notbe used.  Experience has shown that it is vulnerable to variousfailures.  First, if the off period in RFNB is not seen by the sender(due to noise or its being too short), a deadlock occurs and no moredata is transferred.  Second, a two-way receiver cannot talk with astrictly four-way sender, since the sender's next assertion of TYB maydepend on seeing the RFNB transition to on.  And third, the two-wayhandshake is overly sensitive to transitions, and may be activated bynoise pulses. Transitions in the two-way handshake may be missedaltogether in a sender implementation which samples the RFNB line onlyat certain intervals.  The superiority of the more positive four-wayhandshake is important in applications where neither of thecommunicating interfaces is necessarily constructed to particularstandards.2.1.4.  Contact BounceRequirement:Each interface, considered together with the software driving it, mustprevent data from flowing across the interface in either direction whileits Ready relay contacts may be bouncing.  Thus, for 1/10 second afterraising Ready, the outgoing signals There's-Your-Bit andReady-For-Next-Bit must not be asserted.Discussion:This may be accomplished either in hardware or software, as discussed inReport 1822 section B.3.  The delay of 1/10 second is specified here toresolve an ambiguity in Report 1822, concerning whether a shorter delaywas acceptable if the relay was known to solidly finish closing sooner.                                 - 3 -

NWG/RFC# 745                                        MDB2 30-MAR-78 43649JANUS Interface SpecificationsReport 1822 specified a 1/2 second delay, but modern reed relaysreliably finish closing in 1/10 second.2.1.5.  RFNB, TYB Minimum Off TimeRequirement:Ready-For-Next-Bit must be off for at least 50 nanoseconds for localhost connections, and at least 1 microsecond for distant hostconnections, as seen by the receiver of the signal (who is the sender ofdata).  Note that this means that RFNB at the cable driver may have tobe off for somewhat longer than this minimum if deterioration of thesignal waveform along the cable is anticipated.  There's-Your-Bit mustsimilarly be off for at least 50 nanoseconds for local host connections,and at least 1 microsecond for distant host connections, as seen by thereceiver of the signal.Discussion:This extends the Report 1822 requirements for signals received by theIMP, to both interfaces in a JANUS interface pair.2.1.6.  DeskewingRequirement:The outgoing data bit must be on the line and the Last-Bit level correctat least 500 nanoseconds before the sender turns on the There's-Your-Bitsignal.  The sender must turn off TYB before changing either the data orthe LB.Discussion:The responsibility for deskewing signals rests with the sender in eachinterface.  This applies the Report 1822 IMP sender behavior to eachJANUS interface as a requirement.  Note that the receiver may count onthe Last-Bit signal being valid during, and only during, the assertionof There's-Your-Bit.  Specifically, Last-Bit must be asserted duringtransmission of the last data bit.  Report 1822 was slightly ambiguousin this regard.2.1.7.  Transmission OrderRequirement:"The high-order bit of each word is transmitted first."  (Report 1822,section 4.1.)Discussion:If a computer has addressing modes other than word addressing, suchunits or bytes are not used as units of transmission by the interface.For example, the first bit transmitted from or received into a PDP-11 isbit 15, the leftmost bit of a 16-bit word.  This is repeated here tobring it especially to the attention of designers.                                 - 4 -

NWG/RFC# 745                                        MDB2 30-MAR-78 43649JANUS Interface Specifications2.2.  Distant Host Electrical RequirementsDiscussion:The paragraphs below specify a Distant Host option of the JANUSinterface which differs substantially from the 1822 Distant Hostinterface.  Several considerations prompted this change.  Report 1822specifies transformer coupling at the receiver, so requirements onsignal rise time and hold times were made.  To relax these, and toachieve greater tolerance to differences in ground potential, opticalisolators are now often used, even in 1822 interfaces.  Neither theReport 1822 Distant Host driver, nor the driver adopted for JANUS,generate more than 1.0 volt. Commonly available optical isolatorsrequire at least 1.1 volts to overcome their forward drop before theywill operate.  Thus an optical isolator driver is needed in both the1822 and the JANUS receivers.The ground potential difference betweenthe communicating interface may exceed the maximum ratings of the inputamplifier, so the input circuit must be powered from a floating powersupply.  Appropriate DC-DC converters for this purpose are available atreasonable cost.2.2.1.  DH Signal TimingRequirement:Receiver circuits in distant host interfaces shall be implemented withoptical isolators or other means which are not sensitive to rise andhold times, as transformer coupling is.  Therefore, the requirements forrise and hold times on distant host signals appearing in Report 1822 aresuspended.2.2.2.  DH Signal Levels and WaveformsRequirement:Signal levels and waveforms at the driver and the receiver shall followthe specifications in EIA standard RS-422.  In particular, the drivermust supply a differential of at least 2 and not more than 6 volts;  andthe receiver must operate correctly on as small a differential as 0.2volts.2.2.3.  DH Electrical IsolationRequirement:The receiver circuit must operate correctly over a common mode voltagerange of -100 to +100 volts, and must not be permanently damaged by acommon mode voltage of from -300 to +300 volts.                                 - 5 -

NWG/RFC# 745                                        MDB2 30-MAR-78 43649JANUS Interface SpecificationsException:If the interface is absolutely never intended for use in an environmentwhere common mode voltage exceeds 7 volts in magnitude, or where thevoltage from either signal wire to the signal ground exceeds 10 volts inmagnitude, then the electrical isolation required in this paragraph maybe suspended, and the corresponding requirements of EIA specificationRS-422 applied in its place.  Such an implementation is explicitly anexceptional 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 isolatorsuch as the Hewlett-Packard 5082-4360.  The receivers and LED driversfor all input lines may be powered from one source, but this power mustbe floated with respect to ground of the home interface.2.2.4.  DH Cable Shield GroundingRequirement:At each end the cable shield in a distant host connection shall beconnected through a circuit described below to signal ground. Thecircuit consists of two components connected in parallel. (1) A 100K,1/8 watt resistor provides a path to leak off slow accumulations ofstatic 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 notboth!) 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, isundefined when the interface is symmetrical.  Instead, the circuit abovewill be used.2.2.5.  DH CableRequirement:Cable requirements in EIA specification RS-422 must be followed withrespect to quality and electrical characteristics, and those in Report1822 with respect to number of conductors.In particular, at least 10twisted pairs with impedance of approximately 100 ohms must be supplied.                                 - 6 -

NWG/RFC# 745                                        MDB2 30-MAR-78 43649JANUS Interface SpecificationsDiscussion:A suitable cable is PE-39, described in REA Bulletin 345-67. This cableis similar to that mass produced for telephone cable, which is of good,uniform quality, and readily available at reasonable cost.  The cablespecified in Report 1822 is not as desirable.  Note the change inspecified characteristic impedance:  Report 1822 specified 120 ohms,while the JANUS interfaces follow RS-422 with 100 ohms.2.2.6.  DH Cable TerminationRequirement:Termination shall be as specified in RS-422, in particular at thereceiver.  Termination as in Report 1822, at the driver, shall NOT beused.Discussion:The source-end termination specified in Report 1822 was to eliminate thevoltage drop caused by the cable's series resistance.  RS-422 explicitlyallows for this sort of signal attenuation as a part of thespecification.3.  STRONG RECOMMENDATIONS3.1.  Local Host Signal LevelsSuggested voltage levels for local host drivers and receivers are givenbelow.  The levels below are a combination of Report 1822 levels for316/516 and Pluribus machines.  The intent here is to be compatible withreadily available TTL components.  Suggested chips are the 7440 for adriver and the 7420 for a receiver. Note that signals may go up to 6volts, which may damage receiving circuits constructed of normal 5-voltlogic.  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  maxCable impedance and termination circuits are covered in Report 1822.With properly chosen cable and well designed circuits, and withimpedances matched, local host connections may operate considerablyfarther than the 30 feet given in Report 1822. Cables as long as 300feet are in use communicating with ARPANET IMPs.  For example, 300 footcables have worked using 7440's as drivers, standard TTL gates asreceivers, cable termination (on all signal lines) of a diode to groundand a diode to +3 volts, and RG174/U cable.  RG174/U is 50 ohm coax, anda 100 ohm coax is preferred, to reduce ringing.                                 - 7 -

NWG/RFC# 745                                        MDB2 30-MAR-78 43649JANUS Interface Specifications3.2.  Use of the Ready LineIt is strongly recommended that the Ready Line provided by the hardwarebe used by the software in a manner similar or identical to thatdescribed in Report 1822.  Report 1822 sections3.2,4.4 andAppendix Bespecially bear on this topic.  In particular, the software designshould 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 BANDWIDTHIt is advisable to include adjustable delays whose function is to limitthe maximum bandwidth of transfers, as discussed in Report 1822.  Onlywhen the details (such as cable characteristics, memory speed, andacceptable memory utilization) of a specific application guarantee thatan unregulated transfer rate will be acceptable can these delays beomitted.  Two delays are involved, one in the sender circuit and one inthe 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 Cand D in Report 1822 Fig. B-1.)  The receiver delays up to 10microseconds (adjustable) from when the foreign interface assertsThere's-Your-Bit, before again turning on Ready-For-Next-Bit. (This isthe sum of delays A and B in Report 1822 Fig. B-2.) When delivered,interfaces should have these delays set at approximately the maximumdelay.  The timing is shown below.                                 - 8 -

NWG/RFC# 745                                        MDB2 30-MAR-78 43649JANUS Interface Specifications                            _______         _______      sender's TYB  _______!       !_______!       !___                        _______         _______      foreign RFNB  ___!       !_______!       !________                               !<--delay-->!                            _______         _______       foreign TYB  _______!       !_______!       !___                        _______         _______   receiver's RFNB  ___!       !_______!       !________                           !<--delay-->!5.  INTER-OPERABILITY WITH 1822 INTERFACESProtocol specifications have been chosen which are compatible withReport 1822.  Actually, the protocol areas discussed above are furtherclarification of Report 1822, rather than any change from it.  Theelectrical specifications differ only slightly from the 1822 interface.The local host levels chosen are 1822 compatible.  The potentialdifficulties in using a JANUS interface cabled to an 1822 interfacearise with the distant host interface.The distant host cable for a JANUS interface is 100 ohms nominalimpedance, compared to 120 ohms for the 1822 interface.  This differenceis small enough that most applications will work with either cable, oreven with some 100 ohm cable and some 120 ohm cable.The 1822 distant host interface does not provide as much electricalisolation as the standard JANUS distant host interface.  Thus, in casesof severe common mode noise or ground potential difference, two JANUSinterfaces might operate correctly, but an 1822 interface mightmisbehave or burn out.The JANUS distant host driver yields 2 to 6 volts output, and itsreceiver requires 0.2 volts input; the 1822 distant host driver yields1.0 volt output, and its receiver requires 0.1 volt input. Unless thereis a significant signal loss in the cable, the 1822 driver will drive aJANUS receiver acceptably.  On the other hand, the maximum input to an1822 receiver is 4.0 volts.Thus a JANUS driver might overdrive an 1822receiver.  The simplest fix for this is to put a (balanced) voltagedivider at the 1822 receiver, or at the JANUS driver.  The dividershould cut down the maximum voltage from 6 volts to 4 volts, or areduction of 1/3.                                 - 9 -

NWG/RFC# 745                                        MDB2 30-MAR-78 43649JANUS Interface SpecificationsThe above differences are relatively minor, so in most applications aninterconnected 1822 interface and a JANUS interface should operatecorrectly.  Attention must be paid to the electrical isolationsusceptibility of the 1822, and to its maximum input voltage.6.  MILITARY COMPATIBILITYThe EIA specification RS-422 chosen as a base for the JANUS interfacedistant host electrical characteristics is compatible with militaryspecification MIL-188-114.The common mode voltage tolerance of the JANUS interface providessignificant protection against widely varying ground potentials in fieldequipment separated by distances of thousands of feet.7.  REFERENCES"Specifications for the Interconnection of a Host and an IMP," BBNReport 1822, revised January 1976; BBN Inc., 50 Moulton St., Cambridge,Ma. 02138."Electrical Characteristics of Balanced Voltage Digital InterfaceCircuits, 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. ofAgriculture.  Contains specification for PE-39 cable.                                 - 10 -