Atari SX 212 Owner's Manual page 37

SI 5
The bit-mapped SI 5 register sets several communications para¬
meters. In most cases, these parameters can and should be set
by your communications software program. This register is pri¬
marily used for factory testing and is not meant to be used to
control your modem. By default, the modem operates in origi¬
nate mode using full-duplex communication at 1200 bps.
SI 6 to SI 8
These registers are used only for factory testing and should
not be changed.
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