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diff --git a/doc/rfc/rfc745.txt b/doc/rfc/rfc745.txt new file mode 100644 index 0000000..a16095b --- /dev/null +++ b/doc/rfc/rfc745.txt @@ -0,0 +1,590 @@ + +NWG/RFC# 745 MDB2 30-MAR-78 43649 +JANUS Interface Specifications + + + +Network Working Group Michael Beeler +Request for Comments 745 BBN +NIC 43649 30 March 1978 +PRTN 245 + + JANUS Interface Specifications + + (Symmetrical, 1822-like Interface) + +1. INTRODUCTION +1.1. Motivation + +A need arose in the Packet Radio project for specification of an +interface between Packet Radio units and other equipment. This paper is +to meet BBN's responsibility to supply that specification. It is our +hope that it will find application in other areas as well. + +1.2. Historical Relationship to 1822 + +The ARPANET employs a network of switching nodes, called IMPs, to +provide interconnection among user equipment, called hosts. A uniform +means of connecting a host to an IMP is specified in BBN Report Number +1822. Consequently, this interface has become known as an 1822 +interface. + +As the need to interconnect new types of devices has grown, it has +become attractive to implement an 1822-like interface on each end of +pairs of devices which are to communicate. The devices are then +connected electrically, and communication can take place in spite of +differences in processing speed, word length, signal levels and so forth +in 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 symmetrical +interface. Although there is a high degree of symmetry, some aspects +are peculiar to the IMP side and some to the host side. Therefore, two +interfaces constructed to connect to IMPs may not function connected to +each other. In what follows, the unsymmetrical aspects are respecified +in a way which will accomplish full interchangeability. + +The interface specified here is called the JANUS interface, to +distinguish it from the Report 1822 interface. + + + + + + - 1 - + +NWG/RFC# 745 MDB2 30-MAR-78 43649 +JANUS Interface Specifications + + + +1.3. Terminology + +The terms, "IMP" and "host," are not relevant in the present context. +Sections of Report 1822 such as Appendix B are conveniently +re-interpreted by substituting "foreign interface" and "home interface," +respectively. + +2. SPECIFICATIONS + +Report 1822 addresses two aspects of the connection of a host to the +ARPANET, the hardware requirements and the software protocols. Since +the JANUS interface will typically be used in applications other than +connection to the ARPANET, the higher level software protocols are +beyond the scope of this paper. They are properly addressed by +documentation specific to each application. Concern here is only for +electrical specification of the JANUS interface. The various areas +which differ from Report 1822 are as follows. + +2.1. Low-level Protocol + +Certain aspects of the JANUS interface and its operation may be +implemented in hardware, software of a mixture of the two. We refer to +these aspects as "low-level protocol." They are to be distinguished +from such "high-level protocol" aspects as header definitions and data +formats. + +2.1.1. Padding + +Requirement: +Received messages are padded out to a full word (of the home device's +size), 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 do +use 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's +impression that the ARPANET will not implement this aspect of +symmetrical interfaces, so hosts communicating through the ARPANET will +continue to see the marker one-bit appended by the source IMP regardless +of whether the hosts have 1822 or JANUS interfaces. + +2.1.2. Message Length + +Requirement: +A JANUS interface must accept messages up to and including 8160 bits +long. + + + + + - 2 - + +NWG/RFC# 745 MDB2 30-MAR-78 43649 +JANUS Interface Specifications + + + +Exception: +If the interface is absolutely never intended for use in +ARPANET-compatible applications, this requirement may be relaxed in any +of three ways. A smaller maximum length may be implemented; a larger +maximum lengthbe implemented; or the maximum length may be so large as +to be in practice infinite. + +Discussion: +A JANUS interface may discard messages longer than 8160 bits when used +with the ARPANET. This constraint can be enforced in software rather +than in hardware, if desired. + +2.1.3. Four-way Handshake + +Requirement: +The interface must use the four-way handshake. That is, the receiver +must wait until the incoming There's-Your-Bit drops before turning on +Ready-For-Next-Bit. + +Discussion: +The two-way handshake, presented as an option in Report 1822, must not +be used. Experience has shown that it is vulnerable to various +failures. 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 more +data is transferred. Second, a two-way receiver cannot talk with a +strictly four-way sender, since the sender's next assertion of TYB may +depend on seeing the RFNB transition to on. And third, the two-way +handshake is overly sensitive to transitions, and may be activated by +noise pulses. Transitions in the two-way handshake may be missed +altogether in a sender implementation which samples the RFNB line only +at certain intervals. The superiority of the more positive four-way +handshake is important in applications where neither of the +communicating interfaces is necessarily constructed to particular +standards. + +2.1.4. Contact Bounce + +Requirement: +Each interface, considered together with the software driving it, must +prevent data from flowing across the interface in either direction while +its Ready relay contacts may be bouncing. Thus, for 1/10 second after +raising Ready, the outgoing signals There's-Your-Bit and +Ready-For-Next-Bit must not be asserted. + +Discussion: +This may be accomplished either in hardware or software, as discussed in +Report 1822 section B.3. The delay of 1/10 second is specified here to +resolve an ambiguity in Report 1822, concerning whether a shorter delay +was acceptable if the relay was known to solidly finish closing sooner. + + + + - 3 - + +NWG/RFC# 745 MDB2 30-MAR-78 43649 +JANUS Interface Specifications + + + +Report 1822 specified a 1/2 second delay, but modern reed relays +reliably finish closing in 1/10 second. + +2.1.5. RFNB, TYB Minimum Off Time + +Requirement: +Ready-For-Next-Bit must be off for at least 50 nanoseconds for local +host connections, and at least 1 microsecond for distant host +connections, as seen by the receiver of the signal (who is the sender of +data). Note that this means that RFNB at the cable driver may have to +be off for somewhat longer than this minimum if deterioration of the +signal waveform along the cable is anticipated. There's-Your-Bit must +similarly be off for at least 50 nanoseconds for local host connections, +and at least 1 microsecond for distant host connections, as seen by the +receiver of the signal. + +Discussion: +This extends the Report 1822 requirements for signals received by the +IMP, to both interfaces in a JANUS interface pair. + +2.1.6. Deskewing + +Requirement: +The outgoing data bit must be on the line and the Last-Bit level correct +at least 500 nanoseconds before the sender turns on the There's-Your-Bit +signal. The sender must turn off TYB before changing either the data or +the LB. + +Discussion: +The responsibility for deskewing signals rests with the sender in each +interface. This applies the Report 1822 IMP sender behavior to each +JANUS interface as a requirement. Note that the receiver may count on +the Last-Bit signal being valid during, and only during, the assertion +of There's-Your-Bit. Specifically, Last-Bit must be asserted during +transmission of the last data bit. Report 1822 was slightly ambiguous +in this regard. + +2.1.7. Transmission Order + +Requirement: +"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, such +units 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 is +bit 15, the leftmost bit of a 16-bit word. This is repeated here to +bring it especially to the attention of designers. + + + + - 4 - + +NWG/RFC# 745 MDB2 30-MAR-78 43649 +JANUS Interface Specifications + + + +2.2. Distant Host Electrical Requirements + +Discussion: +The paragraphs below specify a Distant Host option of the JANUS +interface which differs substantially from the 1822 Distant Host +interface. Several considerations prompted this change. Report 1822 +specifies transformer coupling at the receiver, so requirements on +signal rise time and hold times were made. To relax these, and to +achieve greater tolerance to differences in ground potential, optical +isolators are now often used, even in 1822 interfaces. Neither the +Report 1822 Distant Host driver, nor the driver adopted for JANUS, +generate more than 1.0 volt. Commonly available optical isolators +require at least 1.1 volts to overcome their forward drop before they +will operate. Thus an optical isolator driver is needed in both the +1822 and the JANUS receivers. The ground potential difference between +the communicating interface may exceed the maximum ratings of the input +amplifier, so the input circuit must be powered from a floating power +supply. Appropriate DC-DC converters for this purpose are available at +reasonable cost. + +2.2.1. DH Signal Timing + +Requirement: +Receiver circuits in distant host interfaces shall be implemented with +optical isolators or other means which are not sensitive to rise and +hold times, as transformer coupling is. Therefore, the requirements for +rise and hold times on distant host signals appearing in Report 1822 are +suspended. + +2.2.2. DH Signal Levels and Waveforms + +Requirement: +Signal levels and waveforms at the driver and the receiver shall follow +the specifications in EIA standard RS-422. In particular, the driver +must supply a differential of at least 2 and not more than 6 volts; and +the receiver must operate correctly on as small a differential as 0.2 +volts. + +2.2.3. DH Electrical Isolation + +Requirement: +The receiver circuit must operate correctly over a common mode voltage +range of -100 to +100 volts, and must not be permanently damaged by a +common mode voltage of from -300 to +300 volts. + + + + + + + + + - 5 - + +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. + + + + + + + + + + - 6 - + +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. + + + + - 7 - + +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. + + + + + + + + + + + + + + + + + + + - 8 - + +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. + + + + + + + - 9 - + +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. + + + + + + + + + + + + + + + + + + + + + + + + + - 10 -
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