1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 8 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU05“,)#G0G0$!4!G0G0.%47/2+33%26)#%3G0G0!.$G0G0 amended at Geneva, 1980 and Melbourne, 1988)The CCITT,I considering, firstly,the agreement between the International Organization for Standardization (ISO) and th
2、e CCITT on the maincharacteristics of a seven-unit alphabet (International Alphabet No. 5) to be used for data transmission and fortelecommunications requirements that cannot be met by the existing five-unit International Telegraph Alphabet No. 2;the interest, both to the users and to the telecommun
3、ication services, of an agreement concerning thechronological order of transmission of bits in serial working;recommendsthat the agreed rank number of the unit in the alphabetical table of combinations should correspond to thechronological order of transmission in serial working on telecommunication
4、 circuits;that, when this rank in the combination represents the order of the bit in binary numbering, the bits should betransmitted in serial working with the low order bit first;that the numerical meaning corresponding to each information unit considered in isolation is that of the digit:0 for a u
5、nit corresponding to condition A (travail = space), and1 for a unit corresponding to condition Z (repos = mark),in accordance with the definitions of these conditions for a two-condition transmission system;II considering, moreover,that it is often desirable, in character oriented data and messages
6、transmission, to add an extra “parity“ unit toallow for the detection of errors in received signals;the possibility offered by this addition for the detection of faults in terminal equipment;the need to reserve the possibility of making this addition during the transmission itself, after the sevenin
7、formation units proper have been sent;recommendsthat signals of International Alphabet No. 5 code for data and messages transmission should in general includean additional “parity“ unit;that the rank of this unit and, hence, the chronological order of the transmission in serial working should bethe
8、eighth of the combination thus completed;_1)See Recommendation V.4 for data transmission over public telephone networks.2 Fascicle VIII.2 - Rec. X.4III consideringthat, in start-stop systems working with electromechanical equipment, the margin of such equipment and thereliability of the connection a
9、re considerably increased by the use of a stop element corresponding to the duration oftwo-unit intervals of the modulation;that for start-stop systems using International Alphabet No. 5 at modulation rates of 200 and 300 bauds,Recommendations X.1 and S.31 specify that transmit devices should use a
10、stop element lasting at least two units;that the previously expressed preference for a two-unit stop element arises from a transmission point of viewwhere anisochronous public data networks are concerned;recommendsthat in start-stop systems using combinations of International Alphabet No. 5 normally
11、 followed by a parityunit, the first information unit of the transmitted combination should be preceded by a start element corresponding tocondition A (space);that the duration of this start element should be a one-unit interval for the modulation rate under consideration,at transmitter output;that
12、the combination of seven information units, normally completed by its parity unit, should be followed bya stop element corresponding to condition Z (mark);that for public anisochronous data networks, data terminal equipment using International Alphabet No. 5should comply with Recommendations X.1 and
13、 S.31 and use a stop element lasting at least two units;that the start-stop receivers should be capable of correctly receiving start-stop signals from a source whichappears to have a nominal cycle of 10 units (i.e., with a nominal one-unit stop element). However, for certainelectromechanical equipme
14、nt the receivers may only be capable of correctly receiving signals when the stop element isnot reduced below one unit (even in the presence of distortion);IV considering, finally,that the direction of the parity unit can only be that of the even parity on the perforated tapes, particularlyowing to
15、the possibility of deletion (combination 7/15 of the alphabet) which causes a hole to appear in all tracks;that, on the other hand, the odd parity is considered essential in the equipment which depends on transitions inthe signals to maintain synchronism in cases where combination 1/6 (SYNC) of the
16、alphabet does not permit aneconomical solution;recommendsthat the parity unit of the signal should correspond to the even parity in links or connections operated on theprinciple of the start-stop system;that this parity should be odd on links or connections using end-to-end character oriented synchr
17、onousoperation;that arrangements should be made when necessary to reverse the direction of the parity unit at the input andoutput of the synchronous equipment connected either to apparatus working on the start-stop principle or receivingcharacters on perforated tape;Fascicle VIII.2 - Rec. X.4 3that
18、the detection of a character out-of-parity may be represented by:a) a reverse question mark graphic character or a representation of the capital letter SB (see ISO 2047)provided that this letter occupies a single character position on a screen or printer, and which could havebeen entered by a single
19、 key stroke. It is recognized that it may be difficult to achieve a legible “SB“character from some matrix printers or displays where the characters are printed; orb) a recording of the 1/10 (SUB) character in the tape or other storage medium, where provided.Where a SUB character occurs in a received transmission, or is presented to a DTE via a storage medium, e.g.paper tape, then the reaction should be as in a) and b) above.
