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USER'S GUIDE
where it is essential to do so. The reason is that there are inherent reasons why recovery
from such an interruption can result in garbled copy at one end of the contact, in particu-
lar in 'figs' garble.
If possible, wait until the other station is idling before breaking in, and if the other station
breaks in to your transmission, it will probably help to use the 'clear buffer' facility if such
exists to abort the later transmission of any unsent text which would be inappropriate to
the new context of the break-in.
3.4.5.
The AMTOR alphabet, like the RTTY alphabet, consists of two sets of 30 characters, with
a switch made between them by two 'shift' or 'case' codes. On inherent result of this tech-
nique is that it is often not known which shift the distant receiving station is in at the
commencement of the contact. For this reason it is always good practice to send the ap-
propriate shift code at the start of each contact, and indeed, at the start of each mes-
sage, and perhaps at more frequent intervals. With terminals encoded in teleprinter code,
there are always two keys labeled 'letters' and 'figures,' and so it is simply necessary to
hit the appropriate one of these keys as required. However, on more modern terminals,
these two keys may not exist, and the sending of the shift code may be 'hidden' from the
user.
However, since the distant terminal could still nevertheless be in the wrong shift, there
will always be the requirement to send the shift code at the start, to prevent the distant
receiver copying the first part of the text in the wrong shift. Consult the documentation
with your AMTOR unit or terminal in order to establish how to do this if there is no 'let-
ters' or 'figures' key.
4. Format of the SELCAL Code
Although the convention is to form the SELCAL from the call sign, some AMTOR units have the
possibility to include any AMTOR character in the SELCAL. It is strongly recommended, how-
ever, that only the 26 letters A-Z are used in SELCAL codes.
PK232UG Rev. E 9/87
AMTOR OPERATING SUGGESTIONS FROM G3PLX
D-6
184
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