Campbell Scientific RF400/RF410/RF415 Spread Spectrum Radio/Modem - Manual (Page 56)

Appendix G. Short-Haul Modems
G-2
With short-haul modems it is necessary to configure the base
station RF400’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 RF400. If
you don’t do this, the base RF400’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.
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