14
TOTE-A-FAX 542
4.4 S-Registers for the Data Modem
4.4.1 W
HAT IS AN
S-R
EGISTER
?
A modem holds vital information
about modem configuration and
operation in internal memory
locations called Status (S-) Registers.
Each S-Register is assigned a number
from 0 to 27, and each contains 8
data bits (or, one byte of
information). In some S-Registers
each bit has a different function;
these are termed bit-mapped S-
Registers. The bits are numbered
0—7, with 0 the least significant bit.
Other S-Registers use the total
number of bits (8) to represent a
single value.
One of the functions of an S-Register
is to store modem configuration
information. Thus, commands
which change or alter the
configuration operate by modifying
the corresponding S-Register.
Most S-Registers have a fixed,
predefined meaning to the modem,
and can be read or altered. Others
are read only registers; if you try to
change that particular register, the
modem responds with an error
message. Still other registers are
either destined not to be used or are
reserved for future options for the
modem. A reserved register cannot
be read or changed.
4.4.2 B
IT
-M
APPED
S-R
EGISTERS
Bit-mapped S-Registers contain
configuration information for more
than one option or modem
function. Each bit is mapped to a
different modem function.
Sometimes a group of bits within a
particular register are mapped to a
function.
4.4.3 D
IFFERENT
S-R
EGISTER
S
ETS AND
T
HEIR
P
URPOSES
The Tote-A-Fax 542 modem uses
three different sets of S-Register
values to do its various functions and
options. You can alter (modify)
some registers but not others.
Here is a summary of the three types
of S-Reigsters:
• Active (Current) Configuration
S-Registers
These are the S-Register values
that the modem uses to operate.
When you “read from” or “write
to” an S-Register, you are
working with an active (current)
configuration S-Register.
