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Recommendation X.4
GENERAL STRUCTURE OF SIGNALS OF INTERNATIONAL ALPHABET No. 5 CODE
FOR CHARACTER ORIENTED DATA TRANSMISSION OVER PUBLIC DATA NETWORKS1)
(Geneva, 1976; amended at Geneva, 1980 and Melbourne, 1988)
The CCITT,
I considering, firstly,
the agreement between the International Organization for Standardization
(ISO) and the CCITT on the main characteristics of a seven-unit alphabet
(International Alphabet No. 5) to be used for data transmission and for
telecommunications requirements that cannot be met by the existing five-unit
International Telegraph Alphabet No. 2;
the interest, both to the users and to the telecommunication services, of
an agreement concerning the chronological order of transmission of bits in serial
working;
recommends
that the agreed rank number of the unit in the alphabetical table of
combinations should correspond to the chronological order of transmission in
serial working on telecommunication circuits;
that, when this rank in the combination represents the order of the bit in
binary numbering, the bits should be transmitted 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 unit c to condition A (travail =
space), and
1 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
transmission, to add an extra "parity" unit to allow 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 seven information units proper have been sent;
recommends
that signals of International Alphabet No. 5 code for data and messages
transmission should in general include an additional "parity" unit;
that the rank of this unit and, hence, the chronological order of the
transmission in serial working should be the eighth of the combination thus
completed;
III considering
that, in start-stop systems working with electromechanical equipment, the
margin of such equipment and the reliability of the connection are considerably
increased by the use of a stop element corresponding to the duration of two-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 stop element lasting at least two units;
that the previously expressed preference for a two-unit stop element
arises from a transmission point of view where anisochronous public data networks
are concerned;
1) See Recommendation V.4 for data transmission over public telephone networks.
Fascicle VIII.2 - Rec. X.4 PAGE1
recommends
that in start-stop systems using combinations of International Alphabet
No. 5 normally followed by a parity unit, the first information unit of the
transmitted combination should be preceded by a start element corresponding to
condition 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 the combination of seven information units, normally completed by its
parity unit, should be followed by a stop element corresponding to condition Z
(mark);
that for public anisochronous data networks, data terminal equipment using
International Alphabet No. 5 should comply with Recommendations X.1 and 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 which appears to have a nominal cycle of 10
units (i.e., with a nominal one-unit stop element). However, for certain
electromechanical equipment the receivers may only be capable of correctly
receiving signals when the stop element is not 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, particularly owing to 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 in the signals to maintain synchronism [in
cases where combination 1/6 (SYNC) of the alphabet does not permit an economical
solution];
recommends
that the parity unit of the signal should correspond to the even parity in
links or connections operated on the principle of the start-stop system;
that this parity should be odd on links or connections using end-to-end
character oriented synchronous operation;
that arrangements should be made when necessary to reverse the direction
of the parity unit at the input and output of the synchronous equipment connected
either to apparatus working on the start-stop principle or receiving characters
on perforated tape;
that 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 have
been entered by a single 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; or
b) 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 transmisson, 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.
PAGE2 Fascicle VIII.2 - Rec. X.4