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At calling time the sequence as described above with the
FSK type modem is valid for the V22(bis). The called
modem reacts instead or after the answer sequence with
unscrambled binary '1's at 1200 bit/sec. and well in the
upper channel (2400 Hz. carrier). This till it detects
scrambled '0' or '1's in the lower channel (1200 Hz.
carrier). The calling modem remains silent till it for
155 msec. detects unscrambled '1's in the upper channel,
after which it remains silent for 456 msec. and then
transmit unscrambled double dibits 00 and 11, if in
V22bis mode, for 100 msec. followed by scrambled binary
'1's or if in V22 mode it transmits direct scrambled
binary '1's (*). Here the distinction 1200/2400 baud is
made by the calling modem. The called modem reacts with
scrambled '1's preceded, if V22bis with the unscrambled
double dibit 00 and 11. At the moment of detection of
the double dibits, the circuit CI/112 (pin 12) is set
true (indicating 2400 baud). In both cases the scrambled
'1's are observed for 600 msec. after which the circuit
CF/109 (pin 8) is set true. If V22 (1200) the data
transfer is started under control of CA/104 (pin 3). In
the V22bis mode (2400), first scrambled binary '1's are
send form both sides for 200 msec. on 2400 bit/sec.
mode. Noted, that till now all was transmitted on 1200
bit/sec. After this 200 msec. also the V22bis mode is
ready for data transfer.
I know it is confusing, but so it works. To make it more
difficult, the (*) mentioned in the text is true for
V22bis and for V22 alternative A and B only. For V22
alternative C, for the '1's you have to read '0's. As
the V22 norm is very confusing on some aspects, one
remark has to be made. In V22 (not V22bis) 3
alternatives can be selected. Alternative A and B are
compatible with V22bis, but alternative C not. The
difference is the scrambled binary '0's stead '1's of
the calling modem. So the real difference between
alternatives A/B and C is, that alternative C includes
the possibility for mode V, for anisochronenous data
upto 300 bit/sec. Which manufacturer tells me which
alternative as mentioned in V22 he serves. Or has this
to do with the Bell-212 norm. Who can tell? Has this to
do with the famous ATBn command in the HAYES-2400 modem?
See above.
Be carefully with these V22 and Bell-212A norms. As
explained before only the 2400 baud (V22bis) is
compatible. The 1200 sometimes works, but sometimes not.
E.g. I had an USA RIXTON 212 modem, not corrected for
the European market. Also I had a Dutch designed (DATAD)
V22(bis) V23 and V21 modem and an English (PACE) for the
same speeds. The RIXTON works pretty most at the time
with both the DATAD and PACE modem. But sometimes it
goes wrong and the Eurpeans connects at 2400, which is
strange, but true. With an original HAYES-1200 (with no
ATBn) command as the HAYES-2400 has, nearly the same
result. The two Europeans hardly are connectable at 1200
baud to each other. This has in my opinion to do with
the way of speed selection. We try to come out of this
struggle.
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