SDM-SIO4 User Guide
2-2
2.2 Selecting RS232 or 5V Logic for Each Port
The output voltage levels of each serial port can be set to either:
• Logic level output: +5V (high) / 0V (low) or
• RS232 output: +5V (high) / -5V (low) (compatible with RS232 driver
requirements).
The logic level output is used when driving a logic level compatible device, e.g.
an SC32A, or where quiescent current is a concern. This mode is lower power
because the idle state is 0V, while the idle state of the RS232 output is -5V
which results in current flow to ground via the input resistance of the RS232
device’s inputs. However, in practice, this logic level output mode can be used
with the majority of RS232 sensors as most accept a logic level drive signal as a
valid input signal.
The output levels are configured via a 4 x 3-way block of jumpers at the back of
the unit. There is one jumper for each port. Text is printed on the circuit board to
indicate the port and the relevant position for the two modes. If the jumper is
fitted on (shorts) the two pins close to the right hand side of the unit, the port
will be in RS232 mode and if the two pins close to the left hand side are shorted,
the port is set to 5V logic.
The inputs to the SDM-SIO4 are compatible with either logic level or RS232
signals.
2.3 Connections to the SDM-SIO4
Connect the SDM port to the datalogger SDM terminals as described under
‘SDM Port’ in Section 1. Use a short, low-capacitance, screened cable. The 12V
supply can normally be taken from the 12V supply input to the datalogger.
Make up cables for the RS232 devices to match the connections shown under
‘SDM-SIO4 Port’ in Table 1 (Section 1). Please refer to your sensor manual for a
description of the required connections, including the handshaking requirements.
To minimise susceptibility to noise, use screened connectors and cables.

2.3.1 Transient Protection and Grounding

The G terminal on the SDM-SIO4 acts both as a ground reference point for
digital communications via the SDM port and also as a protective ground for the
SDM-SIO4. Usually it can be connected back to the datalogger power ground (G
on a CR10/10X). This ground in turn should be connected to the safety ground
for the whole system.
The SDM-SIO4 has protection against electrostatic discharge and induced
transients on all input and output lines. However, the level of protection offered
is limited by the grounding paths within the case. Where long cables are to be
run to remote serial devices, especially in areas prone to lightning, external
lightning protection is recommended on all lines connected to the serial ports
(contact Campbell Scientific for further details).
The possibility of ground loops being set up between the datalogger and the
remote RS232 device via the serial cable should also be considered. Ground
loops formed by secondary return earth paths can lead to various problems
including: