User`s manual

U.S. Robotics I-modem Command Reference
Using the AT Command Set • Page 1-6
Bit-Mapped S-registers
Understanding Bit-Mapped S-registers
Certain registers are "bit-mapped." A bit-mapped register uses one number to describe a
collection of settings. While bit-mapping allows us to pack a lot of information in a small
space, it is complex. Unfortunately, some discussion of binary mathematics is necessary to
explain bit-mapping.
When the I-modem displays the value of an S-register, you see a decimal value between 0
and 255. The I-modem, however, understands the decimal value as a collection of binary
digits (bits). HereÕs how bits are mapped to decimal values:
Each bit can be either on (1) or off (0). Eight bits create 256 unique combinations of 1s
and 0s. Each of the eight bits can be assigned a number corresponding to its position:
b b b b b b b b
7 6 5 4 3 2 1 0