Communication system having an information bus

PHN 9737 1 30.3.81 Communication system having an information busn The invention relates to a communication sys-tem of a type suitable for the tr~nsmission of digital information through a single channel information bus between two or more out of a number of~ asynchronously working, S apparatus coupled to the informa-tion bus, each apparatus comprising at least a digital data processing portion having a clock generator for the generation of a periodic clock-~ignal, and the data processing portion being coupled by means o~ a digital control UIIit to th0 informa~ tion bus in such manner that transmission switches in the control units form an AND-gate circuit with the information bus~ The invention also relates to a control unit for coupling an apparatus to the information bus.

Communication systems of the above type are usually used in situations where a plurality of apparatus shave a transfer medium for intercommunication.

This may be the communication between one or more intelligent terminals and a central computer in a laboratory, but the communicat on may also be for the control of domestic ap liances, for example by means of a so called "home-com puter", audlo and/or ~ideo equipment possibly being coupled to the in~ormation bus both ~or the control of these appliances and for applying te~t data to a tele~ision re ceiver9 these text data having ~een obtained by means of a teletext demodulator, or a ~iewdata terminal, etc.

For the inforrnation bus use may be made of amulti-channel bus through which digital information is transmitted in parallelO When the average information density is su~fi~ ciently low, a single-channel bus will usuaIly be suffi cient, the information being transmitted serially.

A communication system of the above type is ~; ~' :

~ :1 fi ~ PI~ 9737 2 30.3.81 known ~rom the report on the "Second Symposium on Micro Architecture "Euromic-ro"~ 1976, pages 299 - 30L~, R.

Sommer. "Cobus, a firmware controlled data transmission system".

This publication discloses a communication system in which the single-channel bus is in the ~orm of a coaxial cable which, in combination with transmission switches connected thereto, ~orms an ~ND-gate circuit which is suitable ~or the transmission o~ information at a speed of approximately 200 kBaud, which means, minimum waiting periods, priority arbitration, addresses included, a net transmission capacity of approximately 10,000 information words, each consisting of 8 to 10 bits, per second.

As the bus circuit forms an AND-gate circui-t, the bus will only be in the "ON" condition when all transmit switches keep the output in the "ON" position, which, in the case -transis-tors are used as transmission switches, generally implies that the transmission switches do not carry current.

When a pulse is required, the transmission ; switch is rendered conductive by applying a current to its base, its output then going to the l'OFF" state.

As soon as at least ore transmission switch supplies an "OFF" pulse, the bus line is in the "OFF" state, irrespective o~ the positions of other transmission swit ches.

So-called "open collector" transmission switches are used which form a wire-A~D ~unction in combination with the coaxial cable.

The choice between "ON" or "OFF" is optional.

In this article an AND-gate circuit is used in which the bus is in its "ON" state when all the coupled inputs are "ON".

This definition will also be used in the further course of this description.

It is well known that this definition is equivalent to an OR-gate circuit ~or "OFF" signals, the wired-O~ function.

Figure 2 o~ the cited publication shows a sim 5 9 ~ . ~ PHN 9737 3 3O.3.81 plified block-diagram of a microprocessor-controlled "Cobus interface~'~ which, in the art, is usually indicated by the term "control-unit".

The Cobus-interface shown comprises inter alia a receivin~ circuit and an "interference detector".

By means o~ these circuits it is checked whether another control unit simultaneously requests the bus line, by checking the transmitted own addressO A priority rule ensures that the apparatus having the lower address is given priority.

In addition, the control units comprise a "carrier detector", that is to say a circuit for reco~structing the bit clock and a synchronizing circuit.

In the starting phases the bit rate is halved to reduce the effect of the synchronization problems due to inter alia : the signal propagation time over the bus cable.

A carrier detector is usually formed by means of a so-called flywheel oscillator or a phase locked loop oscillator (PLLO).

This requires the use ofa very sta~le clock generator~ which can only be realized by the use o~ a crystal-controlled generator.

With a view to the synchronization each word has a star-ting bit. ~icro-synchronization as well as macro-synchronization occurs on the starting bit and on an.y other set bit~ (page 3OO~ right-hand column9 paragraph 5.2).

As a crystal-controlled clock generator is re quired at the transmitting end and a ~lywheel oscillator or a PLLO at the receiver end 9 the circuits are relatively expensiveO They are suitable for the above-mentioned use in a laboratory but they are unacceptably expensive for use in simple systemsO In the described system it is furthermore impossible to couple apparatus which operate with greatly different nominal speeds to one bus. ~inally~ the close time tolerances require well-defined and therefore steep pulse edges9 so that a coaxial cable rnust be i J~g~ ,, P~ 9737 4 30.3.81 used as the bus line to render it possible to keep stray radiation to the en~ironment at an acceptably low level.

A cheap twisted two-wire sys-tem cannot be used in view o~ the requirements as regards interference imposed by postal and similar authorities.

It is an object o~ the invention to provide acheap communication system in which it is possible to use simple clock generators, ~or exarnple a RC-networ~, to de termine the frequency, a cheap two-wire bus and a means to connect apparatus the norninal speed o~ which di~ers e.g.

one order of magn~tude.

According to the invention, a communication system o~ the type described in the preamble is charac terized by a control unit ha~ing means ~or switching the transmission switch, when an apparatus transmits in~orma tion bits o~er the in~ormation bus, for the transmissiono~ a ~O"-pulse during a portion o~ an in~ormation bitperiod which is greater than hal~ the information bi-t period and equal to more than two ~ull periods of -the cloc~ signal~ and ~or the transmission of a "1" pulse during a portion o~ the bit period which is at least a factor o~ 2.1 shorter than the pulse duration of a "O"pulse~ the in~ormation bus being~ set to a logic "OFF" level by means of the switched transmission switch during a "0"~ or a "1"-pulse.

As both a "O" and a "1" produce a pulse, the leQding edge o~ a pulse may ~unction as the starting point ~or a timing circuit, synchroni~ation betwsen the clock generators o~ transmitting and receiving apparatus 30 being super~luous. A~ter it has detected a pulse edge~ receiving apparatus determines after9 for example 9 approximately 3/4 o~ the bit period whether the pulse had ended or not, that is to say whether it represents a "1" or a "O" O The large di~erence in length between a "1" and a "O"-pulse allows a reliable detection, in spite o~ large timing tolerances which are caused by:

,~ 4 5 9 ~ PXN 9737 5 30~3~ 81 a) the use of RC-clock generators which in practice show a fre~uency tolerance of up to ~ 25 %;

b) the use of pulse edges having edge transition times from approximately Or5 to 1~5 microseconds, small variations in a detection threshold already being capable of causing a tirning inaccuracy of some tenths of microseconds, that is to say sometimes more than one full clock period in the case of a 4 MHz MOS-circuit: and c) propagation delay effects over the bus line, to the same order of magnitude as the durations mentioned sub b).

All bit periods in the system a.re expressed in microseconds, they are approximated by the control units in integer numbers of clock periods, these numbers depending on the nominal value of the clock ~requency of the relevant apparatus, so that in general the actual pulse lengths and the detection moments at receiving apparatus correspond only approximately to the defined periods of time.

~ A number of advantageous embodiments as charac terized in the sub claims wi~l be described in detail by way of example in the descripti.on of the accompanying drawing, In the drawings:

Figure 1 is a simplified block-diagram of a communication system;

Figure 2 shows a schematic sur~ey of the composition of an information package to be transmitted;

Figure 3 is a circuit diagram of a portion of a control unit having transmission switches and a receiving portion, and Figure 4 is a simplified block-diagram of a con~ trol unit according to the invention.

Figure 1 shows a bus-structure having an information bus 1 to which~ at taps 2~ a number of apparatuseach having at least a data processing portion 3 comprising a control unit 4, are coupled.

,.~., .

1 3 ~ ~I t~ 9 ~ PI-~ 9737 6 30.3~81 The Figure shows only two o~ the apparatus 1 to N inclusive, denoted by UNIT~1) and I~NIT(N) 7 respectively.

The actual number is generally much larger, dozens or hundreds of these apparatus may be used.

The embodiment described hereina~ter is based on 21 = 4096 addxessable apparatus, but this is in no way a theoretical or other restriction.

Although~ generally, not all apparatus must be capable of communication with all other apparatus, each apparatus will be occasionally in connection with one or more o~ the other coupled apparatusO Some apparatus will then act as "listeners" only~ not counting occasional call or acknowledge sig~als, such as9 ~or example, a washing machine9 Other apparatus will usually act on]y as a "speaker", as, ~or example, a fir~ and/or burglary pro-tection de~ice.

Other apparatus will sometimes act as "speaker", sometimes as "listener", such as, ~or example, a minicomputer~ a ~iewdata termirlal, etc.

In its most simple form the bus has one sole communication channel in the ~orm of, for example, a light conductor~ a coaxial cable or a pair o~ twisted wires ("twisted pair")PO~ Such a bus system always requires an allocation ; regime. A "speaker" occupying the busline must not be distur~ed by other apparatus, as this would result in a mutilation o~ a transmitted message~ Whether a line is busy can howe~er be easily detected with ~nown means by other apparatus.

The situation becomes more complicated when two ;30 "speakers" request use o~ the bus at the same time, at the same tims bei~g understood to mean simultaneously within~ ~or example, some microsecondsA Although, at ~irst sight, this seems statistically highly improbable, it o~ten happens in ac-tual prac-tice.

It is namely possible that two or more apparat~s want to use the bus at greatly dif~erent instants within the period in which the bus line is occupied by a third apparatus.

As soon as the latter releas 6459 ~ PHN 9737 7 es the bus, the apparatus which are s-tanding-by detect this "simultaneously" and all try simultaneously to get the bus at their disposal.

When an apparatus requires the bus it will start transmitting a message, the message being of the general form shown in Figure 2.

During a testing period 82 the control unit checks whether pulses are absent for a certain period of time and then supplies a starting bit 84. For other apparatus this starting bit has the character of a warning("interrupt"), that an information may be forthcoming.

A mode symbol 86 is transmitted thereafter.

When the "master" knows in which mode the prospective "slave" can receive, the mode symbol corresponding with this mode will generally be transmitted.

All low-mode -~ apparatus listening in stand-by must now release the bus during the whole period of time indicated as a "time slot".

When the bus is not simultaneously re~uested by another apparatus an identification 88 (master address) is thereafter transmitted and thereafter the address 90 of the prospective slave.

At the end of, for example, the "slave bits" the master waits for one bit period for an acknowledge signal indicating that the slave is ready to receive.

When this acknowledge signal does not come, this means that the relevant slave is not connected.

When the master does not know -the mode of the slave then it starts in his own highest mode.

This abs~ ence of the acknowledge signal may then imply that the slave can only receive in a lower mode. The master now restarts the message in a lower mode. Should, finally, no acknoweldge signal be forthcoming even in the lowest mode then the master must conclude that the slave is inaccessible, that is to say that it is not coupled to the bus or that it is switched-off.

Normally the acknowledge signal will come and the rest of the message will be sent. If necessary some ~:36~91 PHN 9737 8 check or control bits 82 -then follow and, finally, the actual information transfer 94. At the end of the available time slot 80 the bus is released again 96.

When two or more apparatus require the bus, the arbitration becomes active at the transmission of the mode symbol.

The following description is based, by way of example, on a bus to which apparatus are coupled which have a number of widely different intrinsic speeds. The invention is in no way limited to this example; much more complicated situations and also simpler arrangements can function in a similar manner. When, for example, only one mode occurs the mode symbol may of course be omitted.

The following may serve as an example:

mode 0: clock frequency 0.55 MHz 25%; mode symbol 0 mode 1: clock frequency 2.2 MHz -~ 25~; mode symbol lO mode 2: clock frequency 4.43 MHz ~ O.l~,mode symbol 110.

The following Table I shows the approximate lengths of bit periods for data information or other bits, as well as the corresponding pulse durations.

- TABLE I 9.11 P:EN 9737 9 3 . 3. 81 ~ q7 . _ ~,~ ~ 0~1 CO^ ~D __ ~ ~ ~ , . ' .' ~rl ~ ~ ~ ra U~ :~ _ _ .. ... _~ ~ tQ ' o ,.~ C~ _ __ I ~Q ~ ~o C~l O ~ ~ rr- ~ :~ U~ H ,~ g o o ¢~ ,Q \~ ~ ~ ~ ~ :~ .__ r- ~ ~ . _ . _ ::

: 25 u~ I u~ ~o ~': U~ o,l ~ o~ :~, o o . _ _ o o o o o ~1 ~ C~ ~U r ,Q ~ --~--~- ~ ~ ~ . . .

6D Q rlr~ ~:) R ,Q h1:1 O rl ~ a) rl ~ ~ o ~a __ . _ : .

, . .

5 9 :fl PHN 9737 ~ 30.3.81 In this example the time slot has been given an average length of 7 milliseconds, with a maximum o~ approximately 10 milliseconds.

The mode symbols have been chosen so that when S two or more apparatus request the bus the lower mode apparatus, that is to say the slower apparatus, gets priority.

As the bus 7 being an AND-circuit is OFF dominant and "O"-pulses are of a considerably longer duration than "1"-pulses, all tolerances included, the bus channel be comes also "O"~dominant.

As soon as an apparatus having, ~or example, mode symbol 10 starts in mode 1 and another apparatus hav ing a mode symbol O simultaneously starts in mode 0, then the mode 1 apparatus will already detect when checkin~ for the first 1 that~ contrary to its expectation~ a O ispresent on the bus and will immediately release the bus in favour of the lower mode apparatus.

Likewise, an apparatus having mode ~ will read, for example, 10 instead of 11(0) when s~multaneously a mode 1 apparatus has startedO When two apparatus of the same mode try to occupy the bus, no distinction can be made on the basis of the mode symbol.

Both apparatus recogni~e the mode symbol as good during the test and continue with the identifica~ tion address, both still testin~ each bit simultaneously.

Also in this case the bus is immediately released when a deviation is de-tected.

This means that in the case of equal modes the apparatus having the lowest address is given priority.

If, for example~ -the address of apparatus 30 A is 10100110 and the address of apparatus B is 101000117 then apparatus A will read a O instead of a 1 when checking the sixth bit, and it will release the bus~ Appara-tus B detects a correct bus behaviour and continues its opera- tion. A must immediately release the bus in order to pre vent (in this example) that its eight bit9 a 0~ would disturb the eight bit of B, a 1.

The apparatus which reads its complete mode sym ~ 59~ P~ 9737 11 3O.3~81 bol and identification address on the bus in an undist,-~rbed manner may now occupy the bus during the remaining part of the time slot.

It ~ill be clear that as soon as one or more apparatus are in the arbitration period a third apparatus that tries to get the bus at a significantly later moment, will detect either pulses of mode symbol and identification addresses or of a subsequent message and must wait until the bus has been free of pulses for some timeO The arbitration is only required in the case of simultaneous or approximately simultaneous requests.

In this example i-t was assumed that the 4~43 MHz apparatus have a cr~stal-controlled clock, for ~xample on the basis of a standard crystal for the televisîon PAL frequency of approximately ~.~6 ~IIz~ while the slower ap paratus opcrate with RC-controlled clock generators, When a mode 2 apparatus transmits a mode 1 or a mode O message the control remains unchanged, but first the clock frequency is simply divided by ~ respectively 8.

~or the mode 2 apparatus the difference in length between a "O"-pulse and a "1"-pulse is approximately a factor of 2~ in this example: for the slower ap~aratus with the wider frequency tolerances even a ~actor o~ approximately 4 has been chosen.

For a proper understanding o~ the operation the data included in Table I are sufficient.

In a practical implementation the various time periods, shown in Table II~ have been chosen b~ means o~ a computer program.

The nominal times ha~e been chos~n so that at the nominal frequencies they always correspond to an integer number of clock periods for each apparatusO Only the first part of Table II will be explainc ed in detail.

When, f`or example~ a mode 1 apparatus wants to start transmitting then a star-ting bit 5 will be transmitted with a nominal length o:E 249.2 microseconds.

After a certain propagation time and a portion of a clock period ~ :~ 6~9:~ P~ 9737 12 3~.3781 of another apparatus this starting bit can be received after appro~imately 1008 microseconds.

Thereafter, after 177.0 + 55.5 microseconds, rhe 1 of the mode symbol is given at the moment 6, with a pulse length of 4.1 ~ 1.8 = 5.9 microseconds.

The listening apparatus receives this pulse with certainty within ~.1 micro-seconds if it can operate in mode 1 and determine approximately 9.9 microseconds af-ter the beginning of the received pulse whether the pulse is a 0 or a 1.

The transmitting apparatus performs this check also at approximately the same moment 7a If this check gives the correct result~ the mode-1 "0"-pulse is given thereafter with a pulse duration of approximately 22 microseconds.

By rneans of the bits indicated as "master-bits" the identification address of the master is sent thereafter but shall be interrupted immediately if the check indicates that another apparatus having a lower address must be given priorityO - TABLE II ~ 1 6~59 ~ .

PHN 9737 13 3073~81 __ _ : _ _ _ ,_ ~ L- ~ c~ ~ ,:r~ O c~ ~ O~ ~ ~ ci~ u~ ~ I . _ . ~_ _ , _ _ lo g æ .~ ~ ~ ~ ~ o ~ ~ O U~ rl a) . ~ ~ O ~, .

~ ~ 00 ~ ~ O 1 5 ~d ~ ~ '` .' ~ ~ ~ ~ ~ ~ 1-- r- ~ ~ ~ ~ ~1 ' ~ . _ C5~ , ~ r- 0~ 20 ~1 ; lo C\l co 1~ O ~ t~ O ~¦ ~ u~ .- u~ r O -~7 ~ CO . ~ ~ CO _~ U~ ~ C~! ~ CS`\ ~I CO ~ .__~ ~ ~- O ~ r- O _ __ ___ L~ __ _ __ . '~ O ~O ~_ ~ ~ . ~ . ~ ~U 1~ 1~ 1~ ~ 1~ ~q 1~ ;t ~ 1 cq l ~1 0 ~ c\l F~ I O ~ cu jl O O O ~ O O O V~l ¢~ S 9 .~ PHN9737 14 30.3.81 U~ ~ ~o U~ ~ ~o U~ . ~o ~ ~ r ~ r r r O ~ ~ O - ~ o C~ O ~ ~O ~ c~ ~O u~ ~ ~ ~ .

~ o~ ~ ;i~ co C~i ~ co ~i 20 ' _ _ ~ ~ ~I ~ _ ~I :

~\i ~ ~ . ~ . ~ , ~U . ~ . , ~\ r u~ ~ ~ ~ ~ ~ u ~ 25 ~D _ r 5- r r r r r ~:` ~ ~ 1~ I I ~ ~: ~ ~ 1 ~ o o ~! 1 !

~: ~ 30 , ~1 ~1 , 01 ~ ~ ~1 ~I hl ~ I æ¦ h ¦ d I ^!

~ P:~ I h l O ~ i O r c~ O I O h l U~l ~q I ~ 1 0 0 0 ~ 1 0 0 0 01 0 0 0 '"" ~ .

PXN9737 15 30.3.81 lD u~ ~ o r~ ~ o o o e~ ~ 1 5 , ~ ,~ . ~ ~ o~ C~ ~ oo ,_ O ~ ~ O 20 oo ~U _ co _~ _ cr~ ~ l- ~ O ~ ~ ~ ~ _i i ~ ~ U~ ~ ~_ ~ O 25 r- _ _ r r r ~H 3D :C ¦ o¦ O~ O I O tQ a~ ~DI ~1 a~ O ~ ~ O ~1 c~ ~4l ~~d ~~l ~d ~ ~ ~1 U~ ~ol ~o ~o ~o ~ o o o ~ ~ 59~ P~ 9737 16 30,3.81 Figure 3 shows an example of a transmitting~ recei~ing portion of a control unit suitable for exciting a symmetrical bus channelO At points 10 and 11 -the unit is connected to the bus.

The point 10 is coupled to ground by a resisto:r 12 and the point 11 is coupled to the supply voltage ~ by a resistor 13.

The bus is also connected to the inputs 14 and 15, respectively, of a differential read amplifier 16 having an output 17.

So, i~ there are no pulses on the bus the input 15 has approximately the le~el of the supply voltage and the input 1l~ has approximately ground level; this is the situation defined as the ON~condition of the bus.

The bus is furthermore connected to outputs 18 and l9~ respecti~ely~ of transmit switches 21 and Z2, respectively, which do not carry current in the rest con dition.' Positive digital signals which are generated bya gate circuit, not shown, are applied to an input 23 of the transmitting portion.

This input is directly coupled to a control input 24 of the transmit switch 21 and to a control input 26 of -the transmit switch 22 by an inverter 25.

As soon as a positive pulse appears at the input 23, the two transmit switches 21, 22 become conductive.

In the example given here these switches are impleme~ted by transistors for which the resistors 13 and 12, respec~ tively~ also form the collector resistors.

As soon as the transmit switches 21, 22 become conduc-tive~ the connecting point 10 rises from ground le vel to slightly less than the suppl~ ~oltage, while the conneeting point 11 is set to ground potential~ This is the OFF~condition for the buso Thus~ the input polarity of the read amplifier 16 reverses its sign and an OFF-signal appears at the out put 17.

The ON-condition o~ the bus can only e~ist when the outputs of all -the transmit switches are in the ON 1 3 ~ ~9 ~ Pl~ 9737 17 30.3081 condition, that is to say the switches are non-conducting~ so that the combinati.on of transmit switches and bus indeed behave as an AND-circuit for ON-signals.

When the output 17 of the read amplifier 16 is at the OFF-level while no positive slgnal is applied to the input 23, this is only possible when the transmit switches in another control circuit are conducting: by means of this it is possible to check whether another apparatus transmits a message, or simultaneously requests the bus.

Figure 4 is a simplified block~diagram of a control unit according to the invention.

From the data-processing portion of an apparatus, information is applied to the control unit on ~he inputs 50, 51, usually in the form of data to be transmitted ap plied to -terminal 50 and a destination address applied to terminal 51.

This connection may9 for example, be formed by a data bus and address bus, respectively 9 of a microprocessor.

The informa-tion is applied to a logic unit 53, which supervises and controls iIl a manner which is known se the information to be transmitted or receivedr A number of ~ixed programming da-ta, as well as, for example, an identi~ication address are stored for this purpose i~ a read-only memory ROM7 PROM or a comparable device 55, which is coupled to the logic unit.

The information to be transmitted is applied by logic unit 53 to a pulse-shaper 567 which is in the form of a logic gate circuit which ensures that the pulses are ~U given the length as required in accordance with Table I or Table IIo To this end the pulse shaper is coupled to a clock generator 57, wh~ch is also the clock for the logic unit 53.

The pulse-shaper 56 applies the pulses with the correct length, counted as an integer number of clock periods, to the transmit switches 59, their outputs 18, 19 being co~nected to -the bus 10 and 11~ respectively. ~or ~ 59~ PHN 9737 18 30~3.81 corresponding elements the same reference numbers have been used as in the preceding Figure~ A signal present at -the bus is applied to a receiver 61, which is coupled to an output buffer circuit 63, which has an output 65 for the transrnission of information to the data-processing portion of the apparatus.

For this control this output buffer 63 is also coupled to the logic unit 53.

The output signals of the pulse-shaper 56 as well as the output signals of the receivar 61 are further more applied to a comparator circuit 67 an output 69 of which is coupled to an input 71 for a stop signal of the logic unit 53.

This comparator circuit 67 produces a stop signal as soon as the output signals of pulse-shaper 56 and receiver 61 differ from each other as a resuit of the *act that another apparatus occupies the bus or because another appara-tus of a higher priority simultaneously requests the bus~ as described in the foregoingO The actual shape and contents of the various gate circuits, logic circuits, s-tores and buffer regis ters are not important for the inventive idea.

By meansof them functions are realized which are comparable to the functions which are also required for other circuits9 such as) for e~ample, the functions o:~ the prior art described in the preamble of the present description~ Any person having normal skill in the art can and will implement these functions as they may be required.

The essence of the invention resides in the choice of the ratio between the leng-th of "O" and "1" pulsesm ~hich ratio is greater than 2.1, for example 2~4;

4 or more~ so that a reliable transmission of information can be warranted~ in spite of considera~le tolerances in the clock frequency of the clock generator 57, of time differences in the edge detection by the receiver 61 of edges having a low value of the steepness and of propaga-tion time differenoes on the bus line.

Thereby it now appears to be possible to perform Y~5~ P~ ~737 19 30.3.81 -the transmission of information with the same speed and reliability as in the prior art, in spite of the fact that, particularly with a ~iew to non-professional domestic uses, extremely cheap means ha~e to be used, considerable savings being obtained by the use of a cheap twisted-pair busline without shielding, simple R~-ooupled clock generators and the omission of expensive synchronization means at the receiver end.

The following results were obtained in practice~ At the chosen length of the time slot, the slo west apparatus can transmit~ in addition to the overhead consisting of the starting bit, mode symbol and addresses, one further information-Byte of, for example, 9 or 12 bits.

At first sight this may seem only little, but it is more than fast enough for processing, for example, information coming from a keyboard, either direct or by indra-red or ultrasonic remote control.

In general, not more than one character will then be supplied e~ery 100 milliseconds (or much longer).

This can be processed in nominally 7 and a maximum of 10 milliseconds.

An apparatus in the mode-1 group can already transmit approximately 16 B~tes in one time slot in ad di-tion to the overhead, and a mode-2 apparatus approxi mately 71 Bytes.

In the latter case the transmission was limited for or~anizational reasons to 2 Bytes - 64 Bytes per message.

~ t a clock fre~uency of 4.43 MHz, as used in this example, the Bytes consisting o~, ~or example~ 8 in ~ormation ~its and one parity bit, can be transmitted at a speed corresponding to 8~5 microseconds per bit7 that is approximately 120 kBaud.

~ or -the chosen length of 64 Bytes, this results in an average speed of approximately 10,000 Bytes per second, including the overhead.

For a longer time slot corresponding to 256 Bytes~ the overhead is practically negligible on a percentage base, so that a transmission speed of approximately 13,000 symbols per second is ob 5 9 1 PHN 9737 20 30.3.81 tained which corresponds to 120 kBaud.

However, use of -the invention is in no way limited to such speeds, the transmission speed for this example was chosen in view of the use of MOS-logic, For edge delays of approximately 1 microsecond, a bit rate of e.g. approximation 500 kBaud can be obtained, and when steeper edges are used a correspondingly higher rate, In the latter case shielding of the bus is in general re~uired with a view to -the restrictions imposed on noise.

The required electronic circuits, ~he transmission switches included, may be incorporated in one single integrated circuit~ ,