IBM 3270 Manual page 238

Appendix H. Planning for 3274 Model 51C Switch Settings
Of the many switches that may be installed on a Model 51C operator panel,
three switch groups require some planning: Transmit Level switches, Loop
Data Speed switches, and Modem Address switches. The Transmit Level
switches control the signal level between the Model 51 C and the host. The
Loop Data Speed switches control the data rate of the loop. The Modem
Address switches determine the address that the host will use to
communicate with the Model 51C.
Only one of these switch groups will be on your Model 51C. Transmit Level
switches are installed on a Model 51C that attaches to a 1200-bps switched
network (U.S. only). Loop Data Speed switches are installed on a Model
51C that attaches to an 8100 Information System or a 4331 Processor loop.
Modem Address switches are installed on a Model 51C with a
greater-than-1200-bps integrated modem that attaches to a nonswitched
network.
The following descriptions tell you how to determine whether your Model
51C has any of these switches, how to determine the proper switch settings,
and what information to pass on to the person who will customize your
Model 51C.
Transmit Level Switches (1200 BPS Integrated Modem,
U.S. and Canada Only)
The Transmit Level switches are installed only on a 3274 Model 51C
Control Unit that has an integrated modem feature (#5501, 5502, 5507, or
5508) that attaches to a switched network. These switches are used to
match the Model 51C transmit level to the data coupler (type CDT, CBS, or
FCC-certified equivalent) that is attached to the termination of the
communication line. Consult the data coupler manufacturer's
representative to determine the proper dBm 1 level for the data coupler. Use
Figure H-1 to determine the correct setting of the Transmit Level switches
for the dBm level obtained from the manufacturer's representative; then
enter this information in the instructions following the figure. Give the
completed instructions to the person who will customize your Model 51C.
Same as decibel (dB), but comparison is to an absolute value of power, 1
milliwatt.
Appendix H. Planning for 3274 Model 51C Switch Settings
H-1