分享
 
 
 

RFC745 - JANUS interface specifications

王朝other·作者佚名  2008-05-31
窄屏简体版  字體: |||超大  

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.

 
 
 
免责声明:本文为网络用户发布,其观点仅代表作者个人观点,与本站无关,本站仅提供信息存储服务。文中陈述内容未经本站证实,其真实性、完整性、及时性本站不作任何保证或承诺,请读者仅作参考,并请自行核实相关内容。
2023年上半年GDP全球前十五强
 百态   2023-10-24
美众议院议长启动对拜登的弹劾调查
 百态   2023-09-13
上海、济南、武汉等多地出现不明坠落物
 探索   2023-09-06
印度或要将国名改为“巴拉特”
 百态   2023-09-06
男子为女友送行,买票不登机被捕
 百态   2023-08-20
手机地震预警功能怎么开?
 干货   2023-08-06
女子4年卖2套房花700多万做美容:不但没变美脸,面部还出现变形
 百态   2023-08-04
住户一楼被水淹 还冲来8头猪
 百态   2023-07-31
女子体内爬出大量瓜子状活虫
 百态   2023-07-25
地球连续35年收到神秘规律性信号,网友:不要回答!
 探索   2023-07-21
全球镓价格本周大涨27%
 探索   2023-07-09
钱都流向了那些不缺钱的人,苦都留给了能吃苦的人
 探索   2023-07-02
倩女手游刀客魅者强控制(强混乱强眩晕强睡眠)和对应控制抗性的关系
 百态   2020-08-20
美国5月9日最新疫情:美国确诊人数突破131万
 百态   2020-05-09
荷兰政府宣布将集体辞职
 干货   2020-04-30
倩女幽魂手游师徒任务情义春秋猜成语答案逍遥观:鹏程万里
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案神机营:射石饮羽
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案昆仑山:拔刀相助
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案天工阁:鬼斧神工
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案丝路古道:单枪匹马
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案镇郊荒野:与虎谋皮
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案镇郊荒野:李代桃僵
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案镇郊荒野:指鹿为马
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案金陵:小鸟依人
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案金陵:千金买邻
 干货   2019-11-12
 
推荐阅读
 
 
 
>>返回首頁<<
 
靜靜地坐在廢墟上,四周的荒凉一望無際,忽然覺得,淒涼也很美
© 2005- 王朝網路 版權所有