IBM 3270 Manual page 234

Appendix G. Planning for 3274 Model 31C Modem Address
Switch Settings
Modem Address Switches (Integrated Modems Only)
The Modem Address switches are installed only on a 3274 Model 31C that
has an integrated modem feature (#5640, 5740, 5840, or 5842) installed.
These switches are located behind a door on the control panel. These
switches must be set to the binary equivalent of either your BSC control
unit address (this is the hexadecimal polling address you obtained from the
system programmer for your response to sequence number 301) or your
SDLC control unit address (this is your response to sequence number 302).
Failure to set the Modem Address switches properly will result in valid data
operations of the 3274 but inability to execute the link problem
determination aid (LPDA) function of the integrated modem.
Use Figure G-1 to convert your sequence number response to its binary
equivalent. For example, if your response to sequence number 301 is 21, the
eight Modem Address switches should be set as shown in Figure G-2.
Enter the Modem Address switch settings .in step 2 of the instructions
following Figure G-2. Give the completed instructions to the person who
will customize the 327 4.
Appendix G. Planning for 3274 Model 31C Modem Address Switch Settings
G-1