power than one of the data logger control-port-based COM ports — typically 6 mA minimum.
This is partly because it generates the correct signal levels, which in itself requires power, and
partly because, in the case of RS-232 signals, the resting state of ~ –6 V driving a nominal RS-232
~3 k load implicitly wastes ~2 mA of current per line.
To avoid excess current waste, the chip can be turned off when not in use, for example, between
polled measurements, simply by closing the serial port, using
SerialClose()
(see
[p. 32]). When running in RS-232 mode with a sensor that only sends data one way
to the data logger, run the SDM-SIO2R in “receive-only mode” as this does not turn on the
output drivers (see
Using the RS-232 link in receive-only mode
If using RS-232 mode, also avoid connecting handshaking lines that are not required to avoid an
unnecessary 2 mA drain per line.
The SDM-SIO2R will start up with its last set of baud rate and protocol settings. This feature is to
protect against brownout conditions where the SDM-SIO2R has reset but the data logger has
not. It does, however, mean that the SDM-SIO2R does not start up in its lowest power mode. To
place the SDM-SIO2R into its lowest power mode, you will need to run the
SerialClose()
instruction.
7.3 Schematic
(p. 37) shows the internal configuration of the SDM-SIO2R when configured in RS-485
or RS-422 full-duplex mode. This shows the internal resistors to aid users trying to test system
interconnections and drive levels.
SDM-SIO2R Serial Input/Output Module with Switched Sensor Power
36
