Campbell Scientific RF401-series and RF430-series Spread Spectrum Data Radios/Modems - Manual (Page 82)

Appendix G. Short-Haul Modems
The remote RF401 is typically powered via the datalogger’s CS I/O port.
Refer to Section 4.2, Power Supplies, for more information about power
supplies.
With short-haul modems it is necessary to configure the base
station RF401’s “RS-232 Auto Power Down Enable” (in the
Advanced Setup \ Interface Parameters menu) to mode "0" which
will maintain the radio's RS-232 port always active. This results
in an additional constant 2 mA current drain by the RF401. If you
don’t do this, the base RF401’s RS-232 port will turn off 30
seconds after activity and the attached SRM-5A (which gets its
power from the port) will not receive any messages from the PC.
Refer to Appendix B, Advanced Setup Menu, for information
about the Advanced Setup menu.
NOTE
G-2